Power storage device and electronic device

ABSTRACT

Provided is a power storage device whose charging and discharging characteristics are unlikely to be degraded by heat treatment or a power storage device that is highly safe against heat treatment. The power storage device includes a positive electrode, a negative electrode, a separator, an electrolyte, and an exterior body. The separator is positioned between the positive electrode and the negative electrode and includes polyphenylene sulfide or cellulosic fiber. The electrolyte includes propylene carbonate, ethylene carbonate, and vinylene carbonate, lithium hexafluorophosphate, and lithium bis(pentafluoroethanesulfonyl)amide. A concentration of lithium hexafluorophosphate with respect to the electrolyte is more than or equal to 0.01 wt % and less than or equal to 1.9 wt % in a weight ratio.

TECHNICAL FIELD

One embodiment of the present invention relates to a power storagedevice and an electronic device.

Note that one embodiment of the present invention is not limited to theabove technical field. One embodiment of the invention disclosed in thisspecification and the like relates to an object, a method, and amanufacturing method. One embodiment of the present invention relates toa process, a machine, manufacture, or a composition of matter.Specifically, examples of the technical field of one embodiment of theinvention disclosed in this specification include a semiconductordevice, a display device, a light-emitting device, a power storagedevice, a memory device, an imaging device, a driving method thereof,and a manufacturing method thereof.

In this specification, the power storage device is a collective termdescribing units and devices having a power storage function. Forexample, a power storage device (also referred to as a secondarybattery) such as a lithium-ion secondary battery, a lithium-ioncapacitor, and an electric double layer capacitor are included in thecategory of the power storage device.

BACKGROUND ART

In recent years, a variety of power storage devices such as lithium-ionsecondary batteries, lithium-ion capacitors, and air batteries have beenactively developed. In particular, demand for lithium-ion secondarybatteries with high output and high energy density has rapidly grownwith the development of the semiconductor industry, for portableinformation terminals such as mobile phones, smartphones, and laptopcomputers, portable music players, and digital cameras; medicalequipment; next-generation clean energy vehicles such as hybrid electricvehicles (HEV), electric vehicles (EV), and plug-in hybrid electricvehicles (PHEV); and the like. The lithium-ion secondary batteries areessential as rechargeable energy supply sources for today's informationsociety.

As described above, lithium-ion secondary batteries have been used for avariety of purposes in various fields. Properties necessary for suchlithium-ion secondary batteries are high energy density, excellent cycleperformance, safety in a variety of operation environments, and thelike.

A lithium-ion secondary battery includes at least a positive electrode,a negative electrode, and an electrolyte (Patent Document 1).

REFERENCE Patent Document

[Patent Document] Japanese Published Patent Application No. 2012-009418

DISCLOSURE OF INVENTION

Power storage batteries that are to be mounted in electronic devicessuch as a wearable device and a portable information terminal need toresist heat treatment performed when the electronic devices areprocessed. Particularly in the case where a housing of the electronicdevice and a lithium-ion power storage battery are integrally formed,the lithium-ion power storage battery needs to have heat resistance to atemperature higher than or equal to the manufacturing temperature of thehousing.

High heat resistance of an electrolyte is necessary for high heatresistance of the power storage device. In order to increase the heatresistance of the electrolyte, it is considered effective to suppressthe decomposition of the electrolyte by heat or to suppress thedecomposition of the electrolyte due to a reaction of the electrolytewith other members by heat. Examples of the reaction between theelectrolyte and other members include a reaction with a positiveelectrode, a negative electrode, a separator, or an exterior body.

In view of the above, an object of one embodiment of the presentinvention is to provide a power storage device whose charging anddischarging characteristics are unlikely to be degraded by heattreatment. Another object of one embodiment of the present invention isto provide a power storage device that is highly safe against heattreatment. Another object of one embodiment of the present invention isto provide a power storage device having high flexibility. Anotherobject of one embodiment of the present invention is to provide a novelpower storage device, a novel electronic device, or the like.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

One embodiment of the present invention is a power storage devicecomprising a positive electrode, a negative electrode, a firstseparator, an electrolyte, and an exterior body. The positive electrodeincludes a positive electrode active material layer and a positiveelectrode current collector. The negative electrode includes a negativeelectrode material layer and a negative electrode current collector. Thefirst separator is positioned between the positive electrode and thenegative electrode. The first separator includes polyphenylene sulfideor cellulosic fiber. The electrolyte includes propylene carbonate,ethylene carbonate, and vinylene carbonate, lithium hexafluorophosphate,and lithium salt expressed by General Formula (G1).

In General Formula (G1), R¹ and R² independently represent fluorine or astraight-chain, branched-chain, or cyclic fluoroalkyl group having 1 to10 carbon atoms.

The power storage device preferably includes a second separator. Thesecond separator is preferably positioned between one or both of thepositive electrode and negative electrode and the exterior body andincludes polyphenylene sulfide or cellulosic fiber.

In the power storage device, the concentration of lithiumhexafluorophosphate with respect to the electrolyte is preferably morethan or equal to 0.01 wt % and less than or equal to 1.9 wt % in aweight ratio.

In the power storage device, the lithium salt represented by GeneralFormula (G1) is preferably lithium bis(pentafluoroethanesulfonyl)amide.

In the power storage device, the positive electrode current collectorpreferably includes aluminum or stainless steel.

One embodiment of the present invention is an electronic devicecomprising the above-mentioned power storage device, a band, a displaypanel, and a housing. The power storage device includes a positiveelectrode lead and a negative electrode lead. The positive electrodelead is electrically connected to the positive electrode. The negativeelectrode lead is electrically connected to the negative electrode. Thepower storage device is embedded in the band. Part of the positiveelectrode lead and part of the negative electrode lead protrude from theband. The power storage device has flexibility. The power storage deviceis electrically connected to the display panel. The display panel isincluded in the housing. The band is connected to the housing. The bandincludes a rubber material.

In the electronic device, the rubber material is preferably fluorinerubber or silicone rubber.

One embodiment of the present invention can provide a power storagedevice whose charging and discharging characteristics are unlikely to bedegraded by heat treatment. One embodiment of the present invention canprovide a power storage device that is highly safe against heattreatment. One embodiment of the present invention can provide a powerstorage device having high flexibility. One embodiment of the presentinvention can provide a novel power storage device, a novel electronicdevice, or the like.

Note that the description of these effects does not preclude theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C illustrate examples of a power storage device andelectrodes.

FIGS. 2A and 2B each illustrate an example of a power storage device.

FIGS. 3A and 3B illustrate examples of a power storage device.

FIGS. 4A and 4B illustrate examples of a power storage device.

FIGS. 5A and 5B illustrate examples of a power storage device.

FIGS. 6A to 6F illustrate examples of embossing.

FIGS. 7A and 7B illustrate examples of a power storage device.

FIG. 8 illustrates an example of a power storage device.

FIG. 9 illustrates an example of a power storage device.

FIGS. 10A to 10C illustrate an example of a method for manufacturing apower storage device.

FIGS. 11A to 11C illustrate an example of a method for manufacturing apower storage device.

FIGS. 12A to 12C illustrate an example of a method for manufacturing apower storage device.

FIG. 13 illustrates an example of a method for manufacturing a powerstorage device.

FIGS. 14A to 14C illustrate examples of an electronic device, a band,and a power storage device.

FIGS. 15A to 15C illustrate examples of a band and a power storagedevice.

FIGS. 16A and 16B illustrate an example of a power storage device.

FIG. 17A to 17C illustrate an example of a method for detecting leakage.

FIGS. 18A and 18B illustrate an example of a power storage device.

FIGS. 19A and 19B illustrate an example of a power storage device.

FIG. 20 illustrates an example of a power storage device.

FIGS. 21A to 21D illustrate a method for manufacturing a power storagedevice.

FIGS. 22A, 22B, 22C1, and 22C2 illustrate an example of a power storagedevice.

FIG. 23 illustrates an example of a power storage device.

FIGS. 24A to 24D illustrate a method for manufacturing a power storagedevice.

FIG. 25 illustrates an example of a power storage device.

FIGS. 26A to 26F each illustrate an example of an electronic device.

FIGS. 27A to 27D each illustrate an example of an electronic device.

FIGS. 28A to 28C each illustrate an example of an electronic device.

FIG. 29 illustrates an example of an electronic device.

FIGS. 30A and 30B each illustrate an example of an electronic device.

FIGS. 31A to 31D each show charge and discharge curves in Example 1.

FIGS. 32A to 32D each show charge and discharge curves in Example 1.

FIGS. 33A to 33D each show charge and discharge curves in Example 1.

FIGS. 34A to 34D each show charge and discharge curves in Example 1.

FIGS. 35A to 35D each show cycle characteristics in Example 1.

FIGS. 36A to 36D each show cycle characteristics in Example 1.

FIGS. 37A to 37D each show cycle characteristics in Example 1.

FIGS. 38A to 38D each show cycle characteristics in Example 1.

FIGS. 39A to 39D each show cycle characteristics in Example 1.

FIGS. 40A to 40D each show cycle characteristics in Example 1.

FIGS. 41A to 41D each show cycle characteristics in Example 1.

FIGS. 42A to 42D each show cycle characteristics in Example 1.

FIGS. 43A and 43B show the relationship between the concentration oflithium hexafluorophosphate and cycle characteristics.

FIGS. 44A to 44D each show charge and discharge curves in Example 2.

FIGS. 45A and 45B each show charge and discharge curves in Example 2.

FIGS. 46A to 46C each show cycle characteristics in Example 2.

FIGS. 47A to 47C each show cycle characteristics in Example 2.

FIGS. 48A to 48C each show cycle characteristics in Example 2 FIGS. 49Ato 49C each show cycle characteristics in Example 2.

FIGS. 50A and 50B show the relationship between the concentration oflithium hexafluorophosphate and cycle characteristics.

FIG. 51 shows an XPS measurement position in Example 3.

FIGS. 52A to 52C each show XPS spectra in Example 3.

FIGS. 53A to 53C each show XPS spectra in Example 3.

FIGS. 54A to 54C each show XPS spectra in Example 3.

FIGS. 55A and 55B each show XPS spectra in Example 3.

FIGS. 56A and 56B show TG-DTA measurement results in Example 3.

FIG. 57 shows TG-DTA measurement results in Example 4.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments will be described in detail with reference to the drawings.It will be readily appreciated by those skilled in the art that modesand details of the present invention can be modified in various wayswithout departing from the spirit and scope of the present invention.Thus, the present invention should not be construed as being limited tothe description in the following embodiments.

Note that in structures of the present invention described below, thesame portions or portions having similar functions are denoted by thesame reference numerals in different drawings, and a description thereofis not repeated. Further, the same hatching pattern is applied toportions having similar functions, and the portions are not especiallydenoted by reference numerals in some cases.

Note that the position, size, range, or the like of each structureillustrated in drawings and the like is not accurately represented insome cases for easy understanding. Therefore, the disclosed invention isnot necessarily limited to the position, size, range, or the like asdisclosed in the drawings and the like.

In this specification and the like, flexibility refers to a property ofan object being flexible and bendable. In other words, it is a propertyof an object that can be changed in form in response to an externalforce applied to the object, and elasticity or restorability to theformer shape is not taken into consideration. A power storage devicehaving flexibility, i.e., a flexible power storage device, can bechanged in shape in response to an external force. A flexible powerstorage device can be used with its shape fixed in a state of beingchanged in shape, can be used while repeatedly changed in shape, or canbe used in a state of not being changed in shape. In this specificationand the like, the inside of an exterior body refers to a regionsurrounded by the exterior body of a power storage battery, in which astructure such as a positive electrode, a negative electrode, an activematerial layer, and a separator, and an electrolyte are stored.

Note that the terms “film” and “layer” can be interchanged with eachother depending on the case or circumstances. For example, the term“conductive layer” can be changed into the term “conductive film” insome cases. Also, the term “insulating film” can be changed into theterm “insulating layer” in some cases.

Embodiment 1

In this embodiment, power storage devices of embodiments of the presentinvention will be described with reference to FIGS. 1A to 1C to FIG. 13.

The power storage device of one embodiment of the present inventionincludes a positive electrode, a negative electrode, a separator, anelectrolyte, and an exterior body.

High heat resistance of an electrolyte is necessary for high heatresistance of the power storage device. In order to increase the heatresistance of the electrolyte, it is considered effective to suppressthe decomposition of the electrolyte by heat or to suppress thedecomposition of the electrolyte due to a reaction of the electrolytewith other members by heat. Examples of the reaction between theelectrolyte and other members include a reaction with a positiveelectrode, a negative electrode, a separator, or an exterior body.

Note that in this specification, an electrolyte means a substance havingelectric conductivity. The electrolyte is not limited to liquid and maybe a gelled one or solid. An electrolyte in a liquid state is referredto as an electrolytic solution in some cases. An electrolytic solutioncan be made by dissolving a solute in a solvent. An electrolyte in asolid state is referred to as a solid electrolyte in some cases.

For example, lithium hexafluorophosphate (LiPF₆) represented by GeneralFormula (100) is widely used as a lithium salt serving as a solute of anelectrolyte. However, lithium hexafluorophosphate is poor in chemicaland thermal stability. For example, a trace of moisture causeshydrolysis reaction to generate HF, which might be a cause ofdeterioration of a power storage device. It is also said that lithiumhexafluorophosphate is decomposed into LiF and PF₅ at high temperature,and PF₅ decomposes the solvent. The solute probably has a low stabilityat high temperature. Lithium hexafluorophosphate has a thermaldecomposition temperature of approximately 154° C. Note that the thermaldecomposition temperature is a temperature at which the weight in apowder state decreases by 5% due to thermal decomposition. The change inweight by thermal decomposition can be shown bythermogravimetry-differential thermal analysis (TG-DTA) and the like.

General Formula (G1) represents the lithium salt used in one embodimentof the present invention.

In General Formula (G1), R¹ and R² independently represent fluorine or astraight-chain, branched-chain, or cyclic fluoroalkyl group having 1 to10 carbon atoms.

In this specification and the like, the fluoroalkyl group refers to agroup in which some or all hydrogen atoms of an alkyl group aresubstituted with fluorine atoms. It is preferable that 60% or more ofhydrogen atoms of the alkyl group be substituted for fluorine atoms. Thefluoroalkyl group may have atoms other than carbon, hydrogen, andfluorine, such as oxygen, sulfur, and nitrogen.

The lithium salt represented by General Formula (G1) has high chemicaland thermal stability. The lithium salt also has high decompositiontemperature and thermal resistance; thus, the thermal resistivity of astorage battery including the lithium salt as a solute can be increased.Moreover, the lithium salt represented by General Formula (G1) containsfluorine having high electronegativity; thus, a fluoroalkylsulfonylgroup shows a strong electron-withdrawing property and the dissociationlevel of a lithium ion is very high. For these reasons, the lithium saltis suitable as an electrolyte solute in a power storage device, such asa lithium ion secondary battery. The more fluorine atom the fluoroalkylgroup has, the more preferable it is.

In General Formula (G1), it is more preferable that R¹ be fluorine or astraight-chain, branched-chain, or cyclic fluoroalkyl group having 1 to7 carbon atoms. In General Formula (G1), it is more preferable that R²be fluorine or a straight-chain, branched-chain, or cyclic fluoroalkylgroup having 1 to 7 carbon atoms. When such a lithium salt is used as anelectrolyte solute, the thermal resistivity of the power storage devicecan be increased.

In General Formula (G1), it is more preferable that R¹ be fluorine or astraight-chain, branched-chain, or cyclic fluoroalkyl group having 1 to5 carbon atoms. In General Formula (G1), it is more preferable that R²be fluorine or a straight-chain, branched-chain, or cyclic fluoroalkylgroup having 1 to 5 carbon atoms. In that case, the dissociation levelof the lithium salt represented by General Formula (G1) is increased. Asa result, the electrolyte can have high ion conductivity. Moreover, themolecular weight of the lithium salt represented by General Formula (G1)is not increased, and the weight of the lithium salt dissolved in asolvent is small. Thus, an increase in viscosity of the electrolyte andthe degradation of battery characteristics can be suppressed. The costcan also be reduced.

Specific structural formulae of the lithium salt represented by GeneralFormula (G1) are shown below. The following is preferably used as asolute: lithium bis(fluorosulfonyl)amide (LiN(FSO₂)₂N, abbreviation:LiFSA) represented by Structural Formula (101), lithiumbis(trifluoromethanesulfonyl)amide (Li(CF₃SO₂)₂N, abbreviation: LiTFSA)represented by Structural Formula (102), lithiumbis(pentafluoroethanesulfonyl)amide (Li(C₂F₅SO₂)₂N, abbreviation:LiBETA) represented by Structural Formula (103), lithium(perfluorobuthanesulfonyl)(trifluoromethanesulfonyl))amide(LiN(C₄F₉SO₂)(CF₃SO₂) represented by Structural Formula (104), and thelike. These materials have high thermal decomposition temperatures.Thus, storage devices including these materials as solutes can havehigher thermal resistance. For example, the melting point and thermaldecomposition temperature of lithium bis(fluorosulfonyl)amide is 140° C.and about 300° C., respectively. The melting point and thermaldecomposition temperature of lithium bis(trifluoromethanesulfonyl)amideis 233° C. and about 380° C., respectively. The melting point andthermal decomposition temperature of lithiumbis(pentafluoroethanesulfonyl)amide is 328° C. and about 350° C.,respectively.

However, the lithium salt represented by General Formula (G1) mightreact with a current collector, leading to corrosion of the currentcollector. The corrosion of the current collector might become a causeof a reduction in battery capacity.

The above described lithium hexafluorophosphate represented byStructural Formula (100) might react with a current collector to form apassivation film on the current collector surface and suppress thecorrosion of the current collector.

For this reason, in one embodiment of the present invention, theelectrolyte solute preferably contains the lithium salt represented byGeneral Formula (G1) and lithium hexafluorophosphate (LiPF₆) representedby Structural Formula (100). Lithium hexafluorophosphate (LiPF₆) canform a passivation film on the current collector surface and suppressthe corrosion of the current collector. Lithium hexafluorophosphate(LiPF₆) is desirably used at an amount sufficient for forming apassivation film on the current collector surface. Moreover, the lithiumsalt represented by General Formula (G1) mainly supplies lithium ionswhich serve as carrier ions, and also has high thermal resistance. Inaddition, the use of the lithium salt represented by General Formula(G1) and lithium hexafluorophosphate (LiPF₆) represented by StructuralFormula (100) can make a power storage device have high thermalresistance. The power storage device is unlikely to have a decrease incapacity and energy density after charging and discharging are repeatedeven when a storage battery is subjected to heat treatment.

To increase the thermal resistance of the power storage device, asolvent contained in the electrolyte preferably has a high boiling pointand low vapor pressure. The solvent preferably has high dielectricconstant and high ability of dissolving a solute. As such a solvent,carbonate can be used. The carbonate means a compound containing atleast one carbonic ester in its molecular structure and includes bothcyclic carbonate and chain carbonate in its category. The chain includesboth straight-chain and branched-chain. As cyclic carbonate, ethylenecarbonate (EC) represented by Structural Formula (301), propylenecarbonate (PC) represented by Structural Formula (302), and vinylenecarbonate (VC) represented by Structural Formula (303) can be used. Theboiling points of ethylene carbonate (EC), propylene carbonate (PC), andvinylene carbonate (VC) are 243° C., 242° C., and 162° C., respectively.Moreover, these carbonates have high thermal resistance and low vaporpressure and thus are preferably used as a solvent.

When graphite (layered graphite) is used as the negative electrode,propylene carbonate (PC) does not form a passivating film on thegraphite surface but is intercalated between graphite layers togetherwith lithium ions, separating part of the graphite layers from agraphite particle in some cases. In view of this, it is preferable tomix an electrolyte with a solvent having a function of forming apassivation film on the graphite surface. Examples of the solvent havinga function of forming a passivation film on the graphite surfaceincludes ethylene carbonate (EC), vinylene carbonate (VC), and the like.In one embodiment of the present invention, propylene carbonate (PC),ethylene carbonate (EC), and vinylene carbonate (VC) are used for theelectrolyte solvent. This can suppress the separation of part of thegraphite layers from a graphite particle.

In one embodiment of the present invention, it is preferable that theelectrolyte solvent contain ethylene carbonate (EC), propylene carbonate(PC), vinylene carbonate (VC) and that the electrolyte solute containthe lithium salt represented by General Formula (G1) and lithiumhexafluorophosphate (LiPF₆).

In a power storage device of one embodiment of the present invention, anelectrolyte formed in the following manner is preferably used: vinylenecarbonate (VC) is mixed with a mixed solution in which ethylenecarbonate (EC) and propylene carbonate (PC) are mixed at a volume ratioof 1:1, and lithium bis(pentafluoroethanesulfonyl)amide (LiBETA) andlithium hexafluorophosphate are dissolved in the solution.

Specifically, the amount of vinylene carbonate (VC) dissolved in theelectrolyte is more than or equal to 0.1 wt % and less than or equal to5.0 wt %, preferably 1.0 wt % in the weight ratio. The amount of lithiumbis(pentafluoroethanesulfonyl)amide (LiBETA) dissolved in theelectrolyte is more than or equal to 0.1 mol/L and less than or equal to5.0 mol/L, preferably 1 mol/L in the molecular concentration. The amountof lithium hexafluorophosphate with respect to the electrolyte ispreferably more than or equal to 0.01 wt % and less than or equal to 1.9wt %, further preferably, 0.05 wt % and less than or equal to 1.2 wt %,still further preferably 0.1 wt % and less than or equal to 0.8 wt % inthe weight ratio.

The composition of electrolyte can be examined by X-ray photoelectronspectroscopy (XPS), gas chromatography mass spectrometry (GC-MS), liquidchromatography mass spectrometry (LC-MS), ion chromatography (IC),inductively coupled plasma atomic emission spectroscopy (ICP-AES),atomic absorption spectrometry (AAS), glow discharge mass spectrometry(GD-MS), nuclear magnetic resonance (NMR), Fourier transform infraredspectroscopy (FT-IR), and the like.

Polyethylene, polypropylene, and the like, which are generally used as aseparator, are sensitive to heat. Minute pores of a separator might beblocked at high temperature, resulting in malfunction of the powerstorage device.

In view of the above, a separator containing polyphenylene sulfide (PPS)or cellulosic fiber is preferably used for the power storage device ofone embodiment of the present invention.

The separator containing polyphenylene sulfide and the separatorcontaining solvent-spun regenerated cellulosic fiber have high heatresistance and high chemical resistance.

Moreover, the separator containing polyphenylene sulfide and theseparator containing solvent-spun regenerated cellulosic fiber have lowreactivity to the electrolytic solution at high temperature. Thus,degradation of the output characteristics and the charge and dischargecycle performance can be inhibited.

<Structural Example of Power Storage Device>

Next, a specific structure of the power storage device of one embodimentof the present invention will be described below.

FIG. 1A illustrates a power storage device 500, which is a power storagedevice of one embodiment of the present invention. Although FIG. 1Aillustrates a mode of a thin power storage device as an example of thepower storage device 500, one embodiment of the present invention is notlimited to this example.

As illustrated in FIG. 1A, the power storage device 500 includes apositive electrode 503, a negative electrode 506, a first separator 507,a second separator 520, and an exterior body 509. The power storagedevice 500 may include a positive electrode lead 510 and a negativeelectrode lead 511. A bonding portion 518 corresponds to athermocompression bonding portion in the outer region of the exteriorbody 509.

FIG. 1B illustrates the appearance of the positive electrode 503. Thepositive electrode 503 includes the positive electrode current collector501 and the positive electrode active material layer 502.

As illustrated in FIG. 1B, the positive electrode 503 preferablyincludes the tab region 281. The positive electrode lead 510 ispreferably welded to part of the tab region 281. The tab region 281preferably includes a region where the positive electrode currentcollector 501 is exposed. When the positive electrode lead 510 is weldedto the region where the positive electrode current collector 501 isexposed, contact resistance can be further reduced. Although FIG. 1Billustrates the example where the positive electrode current collector501 is exposed in the entire tab region 281, the tab region 281 maypartly include the positive electrode active material layer 502.

FIG. 1C illustrates the appearance of the negative electrode 506. Thenegative electrode 506 includes the negative electrode current collector504 and the negative electrode active material layer 505.

As illustrated in FIG. 1C, the negative electrode 506 preferablyincludes the tab region 282. The negative electrode lead 511 ispreferably welded to part of the tab region 282. The tab region 282preferably includes a region where the negative electrode currentcollector 504 is exposed. When the negative electrode lead 511 is weldedto the region where the negative electrode current collector 504 isexposed, contact resistance can be further reduced. Although FIG. 1Cillustrates the example where the negative electrode current collector504 is exposed in the entire tab region 282, the tab region 282 maypartly include the negative electrode active material layer 505.

As shown in FIG. 1A, the first separator 507 includes a regionoverlapping with the positive electrode 503 and the negative electrode506. The second separator 520 includes a region overlapping with the tabregions 281 and 282. Note that the second separator 520 is notnecessarily provided.

FIGS. 2A and 2B each illustrate an example of a cross-sectional viewalong dashed-dotted line A1-A2 in FIG. 1A. FIGS. 2A and 2B eachillustrate a cross-sectional structure of the power storage device 500that is formed using a pair of the positive electrode 503 and thenegative electrode 506.

As illustrated in FIGS. 2A and 2B, the power storage device 500 includesthe positive electrode 503, the negative electrode 506, the firstseparator 507, the second separator 520, an electrolyte 508, and theexterior body 509. The first separator 507 is located between thepositive electrode 503 and the negative electrode 506. The secondseparator 520 is located between the positive electrode 503 and theexterior body 509 and between the negative electrode 506 and theexterior body 509. The exterior body 509 is filled with the electrolyte508.

The positive electrode 503 includes a positive electrode active materiallayer 502 and a positive electrode current collector 501. The negativeelectrode 506 includes a negative electrode active material layer 505and a negative electrode current collector 504. The active materiallayer is formed on one surface or opposite surfaces of the currentcollector. The first separator 507 is positioned between the positiveelectrode current collector 501 and the negative electrode currentcollector 504.

The power storage device includes one or more positive electrodes andone or more negative electrodes. For example, the power storage devicecan have a layered structure including a plurality of positiveelectrodes and a plurality of negative electrodes.

The positive electrode 503 and the negative electrode 506 preferablyinclude tab regions so that a plurality of stacked positive electrodescan be electrically connected to each other and a plurality of stackednegative electrodes can be electrically connected to each other.Furthermore, a lead is preferably electrically connected to the tabregion.

FIGS. 3A and 3B are examples of cross-sectional views taken alongdashed-dotted lines B3-B4 and B5-B6 in FIG. 1A, respectively. Thecross-sectional view taken along dashed-dotted line B3-B4 corresponds toa region including the positive electrode lead 510 and the positiveelectrode 503. The cross-sectional view taken along dashed-dotted lineB5-B6 corresponds to a region including the negative electrode lead 511and the negative electrode 506. FIGS. 3A and 3B illustratecross-sectional structures of the power storage device 500 formed usinga pair of the positive electrode 503 and the negative electrode 506.

As shown in FIG. 3A, the second separator 520 is provided between thepositive electrode 503 and the exterior body 509. The second separator520 preferably includes a region overlapping with the tab region 281included in the positive electrode and with the positive electrode lead510. As shown in FIG. 3B, the second separator 520 is provided betweenthe negative electrode 506 and the exterior body 509. The secondseparator 520 preferably includes a region overlapping with the tabregion 282 included in the negative electrode and with the negativeelectrode lead 511.

When a conductive material is used for the outer surface of the exteriorbody 509, and an insulating resin is used for the inner side (thepositive electrode and negative electrode side) of the exterior body509, the resin is melted by heat treatment and the conductive materialon the outer side is exposed in some cases. When the conductive materialof the exterior body is in contact with the positive electrode lead 510,the negative electrode lead 511, the positive electrode currentcollector 501, or the negative electrode current collector 504, leakagemight occur. When the second separator 520 is provided between thepositive electrode 503 and the exterior body 509 and between thenegative electrode 506 and the exterior body 509, the leakage can besuppressed. Note that the second separator 520 is not necessarilyprovided.

FIG. 4A illustrates another example of a cross-sectional view alongdashed-dotted line A1-A2 in FIG. 1A. FIG. 4B is a cross-sectional viewalong dashed-dotted line B1-B2 in FIG. 1A.

FIGS. 4A and 4B each illustrate a cross-sectional structure of the powerstorage device 500 that is formed using a plurality of pairs of thepositive and negative electrodes 503 and 506. There is no limitation onthe number of electrode layers of the power storage device 500. In thecase of using a large number of electrode layers, the power storagedevice can have high capacity. In contrast, in the case of using a smallnumber of electrode layers, the power storage device can have a smallthickness and high flexibility.

The examples in FIGS. 4A and 4B each include two positive electrodes 503in each of which the positive electrode active material layer 502 isprovided on one surface of the positive electrode current collector 501;two positive electrodes 503 in each of which the positive electrodeactive material layers 502 are provided on opposite surfaces of thepositive electrode current collector 501; and three negative electrodes506 in each of which the negative electrode active material layers 505are provided on opposite surfaces of the negative electrode currentcollector 504. In other words, the power storage device 500 includes sixpositive electrode active material layers 502 and six negative electrodeactive material layers 505. Note that although the first separator 507has a bag-like shape in the examples illustrated in FIGS. 4A and 4B, thepresent invention is not limited to this example and the first separator507 may have a stripe shape or a bellows shape.

Alternatively, one positive electrode in which both surfaces of thepositive electrode current collector 501 are provided with the positiveelectrode active material layers 502 in FIGS. 4A and 4B is preferablyreplaced with two positive electrodes in each of which one surface ofthe positive electrode current collector 501 is provided with thepositive electrode active material layer 502. Similarly, one negativeelectrode in which both surfaces of the negative electrode currentcollector 504 are provided with the negative electrode active materiallayers 505 is preferably replaced with two negative electrodes in eachof which one surface of the negative electrode current collector 504 isprovided with the negative electrode active material layer 505. In thepower storage device 500 in FIGS. 5A and 5B, surfaces of the positiveelectrode current collectors 501 on the side not provided with thepositive electrode active material layer 502 face and are in contactwith each other, and surfaces of the negative electrode currentcollectors 504 on the side not provided with the negative electrodeactive material layer 505 face and are in contact with each other. Sucha structure allows the interface between the two positive electrodecurrent collectors 501 and the two negative electrode current collectors504 to serve as sliding planes when the power storage device 500 iscurved, relieving stress caused in the power storage device 500.

Although FIG. 1A illustrates the example where the ends of the positiveelectrode 503 and the negative electrode 506 are substantially alignedwith each other, part of the positive electrode 503 may extend beyondthe end of the negative electrode 506.

In the power storage device 500, the area of a region where the negativeelectrode 506 does not overlap with the positive electrode 503 ispreferably as small as possible.

In the example illustrated in FIG. 2A, the end of the negative electrode506 is located inward from the end of the positive electrode 503. Withthis structure, the entire negative electrode 506 can overlap with thepositive electrode 503 or the area of the region where the negativeelectrode 506 does not overlap with the positive electrode 503 can besmall.

The areas of the positive electrode 503 and the negative electrode 506in the power storage device 500 are preferably substantially equal. Forexample, the areas of the positive electrode 503 and the negativeelectrode 506 that face each other with the first separator 507therebetween are preferably substantially equal. For example, the areasof the positive electrode active material layer 502 and the negativeelectrode active material layer 505 that face each other with the firstseparator 507 therebetween are preferably substantially equal.

In the example illustrated in FIG. 2B, the end of the positive electrode503 is located inward from the end of the negative electrode 506. Withthis structure, the entire positive electrode 503 can overlap with thenegative electrode 506 or the area of the region where the positiveelectrode 503 does not overlap with the negative electrode 506 can besmall. In the case where the end of the negative electrode 506 islocated inward from the end of the positive electrode 503, a currentsometimes concentrates at the end portion of the negative electrode 506.For example, concentration of a current in part of the negativeelectrode 506 results in deposition of lithium on the negative electrode506 in some cases. By reducing the area of the region where the positiveelectrode 503 does not overlap with the negative electrode 506,concentration of a current in part of the negative electrode 506 can beinhibited. As a result, for example, deposition of lithium on thenegative electrode 506 can be inhibited, which is preferable.

As shown in FIGS. 4A and 4B, the end portion of the positive electrode503 can be positioned on an inner side than the negative electrode 506even when a plurality of positive electrodes 503 and the negativeelectrodes 506 is used. The end portion of the positive electrode 503may be substantially aligned with the end portion of the negativeelectrode 506. The end portion of the negative electrode 506 may bepositioned on an inner side than the positive electrode 503.

As illustrated in FIG. 1A, the positive electrode lead 510 is preferablyelectrically connected to the positive electrode 503. Similarly, thenegative electrode lead 511 is preferably electrically connected to thenegative electrode 506. The positive electrode lead 510 and the negativeelectrode lead 511 are exposed to the outside of the exterior body 509so as to serve as terminals for electrical contact with an externalportion.

The positive electrode current collector 501 and the negative electrodecurrent collector 504 can double as terminals for electrical contactwith an external portion. In that case, the positive electrode currentcollector 501 and the negative electrode current collector 504 may bearranged such that part of the positive electrode current collector 501and part of the negative electrode current collector 504 are exposed tothe outside of the exterior body 509 without using electrode leads.

Note that part of a surface of the exterior body 509 preferably hasprojections and depressions. This can relieve stress applied to theexterior body 509 when the power storage device 500 is curved. Thus, thepower storage device 500 can have high flexibility. Such projections anddepressions can be formed by embossing the exterior body 509 before thepower storage device 500 is assembled.

Here, embossing, which is a kind of pressing, will be described.

FIGS. 6A to 6F are cross-sectional views illustrating examples ofembossing. Note that embossing refers to processing for formingunevenness on a film by bringing an embossing roll whose surface hasunevenness into contact with the film with pressure. Note that theembossing roll is a roll whose surface is patterned.

FIG. 6A illustrates an example where one surface of a film 50 used forthe exterior body 509 is embossed.

FIG. 6A illustrates the state where a film 50 is sandwiched between anembossing roll 53 in contact with the one surface of the film and a roll54 in contact with the other surface and the film 50 is transferred in adirection 60. The surface of the film is patterned by pressure or heat.

Processing illustrated in FIG. 6A is called one-side embossing, whichcan be performed by a combination of the embossing roll 53 and the roll54 (a metal roll or an elastic roll such as a rubber roll).

FIG. 6B illustrates the state where a film 51 whose one surface isembossed is sandwiched between the embossing roll 53 and the roll 54 andis transferred in the direction 60. The embossing roll 53 rolls along anon-embossed surface of the film 51; thus, both surfaces of the film 51are embossed. As described here, one film can be embossed more thanonce.

FIG. 6C is an enlarged view of a cross section of a film 52 whose bothsurfaces are embossed. Note that H₁ represents the thickness of the filmin depressions or projections, and H₂ represents the thickness of thefilm at a boundary portion between a depression and its adjacentprojection or the thickness of the film at a boundary portion between aprojection and its adjacent depression. The thickness of the film is notuniform, and H₂ is smaller than H₁.

FIG. 6D illustrates another example where both surfaces of a film areembossed.

FIG. 6D illustrates the state where the film 50 is sandwiched betweenthe embossing roll 53 in contact with one surface of the film and anembossing roll 55 in contact with the other surface and the film 50 isbeing transferred in the direction 60.

FIG. 6D illustrates a combination of the embossing roll 53 and theembossing roll 55, which are a couple of embossing rolls. The surface ofthe film 50 is patterned by alternately provided projections anddepressions for embossing and debossing part of the surface of the film50.

FIG. 6E illustrates the case of using the embossing roll 53 and anembossing roll 56 whose protrusions have a pitch different from that ofprotrusions of the embossing roll 55 in FIG. 6D. Note that a protrusionpitch or an embossing pitch is the distance between the tops of adjacentprotrusions. For example, a distance P in FIG. 6E is a protrusion pitchor an embossing pitch. FIG. 6E illustrates the state where the film 50is sandwiched between the embossing roll 53 and the embossing roll 56and is transferred in the direction 60. The film processed using theembossing rolls with different protrusion pitches can have surfaces withdifferent embossing pitches.

FIG. 6F illustrates the state where the film 50 is sandwiched between anembossing roll 57 in contact with one surface of the film and anembossing roll 58 in contact with the other surface and the film 50 istransferred in the direction 60.

Processing illustrated in FIG. 6F is called tip-to-tip both-sideembossing performed by a combination of the embossing roll 57 and theembossing roll 58 that has the same pattern as the embossing roll 57.The phases of the projections and depressions of the two embossing rollsare coordinate, so that substantially the same pattern can be formed onboth surfaces of the film 50. Unlike in the case of FIG. 6F, embossingmay be performed without coordinating the phases of the projections anddepressions of the same embossing rolls.

An embossing plate can be used instead of the embossing roll.Furthermore, embossing is not necessarily employed, and any method thatallows formation of a relief on part of the film can be employed.

FIG. 7A illustrates an example of the power storage device 500 using anexterior body 529 having projections and depressions formed by theembossing described above. FIG. 7B is a cross-sectional view taken alongthe dashed-dotted line H1-H2 in FIG. 7A. The structure of FIG. 7Bwithout the exterior body 529 is similar to the structure of FIG. 4B.

The projections and depressions of the exterior body 529 are formed soas to include a region overlapping with the positive electrode 503 andthe negative electrode 506. In FIG. 7A, the bonding portion 518 does nothave projections and depressions, but may have projections anddepressions.

Furthermore, the projections and depressions of the exterior body 529are formed at regular intervals in the long axis direction of the powerstorage device 500 (the Y direction in FIG. 7A). In other words, onedepression and one projection are formed so as to extend in the shortaxis direction of the power storage device 500 (the X direction in FIG.7A). Such projections and depressions can relieve stress applied whenthe power storage device 500 is curved in the long axis direction.

Note that the projections and depressions of the exterior body 529 maybe formed so as to have a geometric pattern in which diagonal lines intwo directions cross each other (see FIG. 8). Such projections anddepressions can relieve stresses caused by curving the power storagedevice 500 in at least two directions.

Although the positive electrode lead 510 and the negative electrode lead511 are provided on the same side of the power storage device 500 inFIG. 1A, the positive electrode lead 510 and the negative electrode lead511 may be provided on different sides of the power storage device 500as illustrated in FIG. 9. The electrode leads of the power storagedevice of one embodiment of the present invention can be freelypositioned as described above; therefore, the degree of freedom indesign is high. Accordingly, a product including the power storagedevice of one embodiment of the present invention can have a high degreeof freedom in design. Furthermore, a yield of products each includingthe power storage device of one embodiment of the present invention canbe increased.

<Example of Manufacturing Method for Power Storage Device>

Next, an example of a manufacturing method for the power storage device500, which is a power storage device of one embodiment of the presentinvention, will be described with reference to FIGS. 10A and 10B to FIG.13.

First, the positive electrode 503, the negative electrode 506, and thefirst separator 507 are stacked. Specifically, the first separator 507is positioned over the positive electrode 503. Then, the negativeelectrode 506 is positioned over the first separator 507. In the case ofusing two or more positive electrode-negative electrode pairs, anotherseparator 507 is positioned over the negative electrode 506, and then,the positive electrode 503 is positioned. In this manner, the positiveelectrodes 503 and the negative electrodes 506 are alternately stackedand separated by the first separator 507.

Alternatively, the first separator 507 may have a bag-like shape. Theelectrode is preferably surrounded by the first separator 507, in whichcase the electrode is less likely to be damaged during a fabricatingprocess.

First, the positive electrode 503 is positioned over the first separator507. Then, the first separator 507 is folded along a broken line in FIG.10A so that the positive electrode 503 is sandwiched by the firstseparator 507. Although the example where the positive electrode 503 issandwiched by the first separator 507 is described here, the negativeelectrode 506 may be sandwiched by the first separator 507.

Here, the outer edges of the first separator 507 outside the positiveelectrode 503 are bonded so that the first separator 507 has a bag-likeshape (or an envelope-like shape). The bonding of the outer edges of thefirst separator 507 can be performed with the use of an adhesive or thelike, by ultrasonic welding, or by thermal fusion bonding.

Next, the outer edges of the first separator 507 are bonded by heating.Bonding portions 514 are illustrated in FIG. 10A. In such a manner, thepositive electrode 503 can be covered with the first separator 507.

Note that in the case where the outer edges of the first separator 507are bonded using an adhesive or the like, the amount of the adhesive ispreferably small. The outer edges of the first separator 507 are bondedsuch that an electrode (the positive electrode 503 in FIG. 10A)sandwiched between facing portions of the first separator 507 does notprotrude from the first separator 507; thus, for example, when thebonding portions 514 are formed as illustrated in FIG. 10B, the amountof the adhesive can be reduced. In FIG. 10B, the bonding portions 514are formed at the following portions of the outer edges of the firstseparator 507: portions of two sides intersecting with a side where afold is formed that are close to the fold; and a portion of a sideopposite to the side where the fold is formed.

Then, the negative electrodes 506 and the positive electrodes 503 eachcovered with the separator are alternately stacked as illustrated inFIG. 10C. Furthermore, the positive electrode lead 510 and the negativeelectrode lead 511 each having a sealing layer 115 are prepared.Thermoplastic resin such as polypropylene and the like can be used forthe sealing layer 115.

After that, the positive electrode lead 510 having the sealing layer 115is connected to the tab region 281 of the positive electrode 503 asillustrated in FIG. 11A. FIG. 11B is an enlarged view of a connectionportion. The tab region 281 of the positive electrode 503 and thepositive electrode lead 510 are electrically connected to each other byirradiating the bonding portion 512 with ultrasonic waves while applyingpressure thereto (ultrasonic welding). In that case, a curved portion513 is preferably provided in the tab region 281.

This curved portion 513 can relieve stress due to external force appliedafter fabrication of the power storage device 500. Thus, the powerstorage device 500 can have high reliability.

The negative electrode lead 511 can be electrically connected to the tabregion 282 of the negative electrode 506 by a similar method.

Subsequently, the positive electrode 503, the negative electrode 506,and the first separator 507 are positioned over the second separator520.

Next, the second separator 520 is folded along a broken line in FIG. 11Cso that the positive electrode 503, the negative electrode 506, and thefirst separator 507 are sandwiched by the second separator 520. Notethat the second separator 520 preferably covers the tab regions 281 and282.

Here, it is preferable that the outer edges of the second separator 520be bonded so that the second separator 520 has a bag-like shape (or anenvelope-like shape). The bonding of the outer edges of the secondseparator 520 can be performed with the use of an adhesive or the like,by ultrasonic welding, or by thermal fusion bonding.

Next, the outer edges of the second separator 520 are bonded by heating.FIG. 12A shows a bonding portion 521. In such a manner, the positiveelectrode 503, the negative electrode 506, and the first separator 507can be covered with the second separator 520.

Note that in the case where the outer edges of the second separator 520are bonded using an adhesive or the like, the amount of the adhesive ispreferably small. The outer edges of the second separator 520 are bondedsuch that the positive electrode 503 and the negative electrode 506which are sandwiched between the second separator 520 and the firstseparator 507 do not protrude from the second separator 520; thus, forexample, when the bonding portions 521 are formed as illustrated in FIG.12B, the amount of the adhesive can be reduced. In FIG. 12B, the bondingportions 521 are formed at the following portions of the outer edges ofthe second separator 520: portions of two sides that intersect with aside where a fold is formed that are close to the fold; and the vicinityof the tab regions 281 and 282.

Note that the second separator 520 is not necessarily provided. If thesecond separator 520 is not provided, the process related to the secondseparator 520 can be eliminated.

Subsequently, the positive electrode 503, the negative electrode 506,the first separator 507, and the second separator 520 are positionedover an exterior body 509.

Then, the exterior body 509 is folded along a portion shown by a dottedline in the vicinity of a center portion of the exterior body 509 inFIG. 12C.

In FIG. 13, the thermocompression bonding portion in the outer edges ofthe exterior body 509 is illustrated as a bonding portion 118. Then, theouter edges of the exterior body 509 except an inlet 119 for introducingthe electrolyte 508 are bonded by thermocompression bonding. Inthermocompression bonding, sealing layers provided over the leads arealso melted, thereby fixing the leads and the exterior body 509 to eachother. Moreover, adhesion between the exterior body 509 and the leadscan be increased.

After that, in a reduced-pressure atmosphere or an inert gas atmosphere,a desired amount of electrolyte 508 is introduced to the inside of theexterior body 509 from the inlet 119. Lastly, the inlet 119 is sealed bythermocompression bonding. Through the above steps, the power storagedevice 500, which is a thin power storage device, can be fabricated.

Aging may be performed after fabrication of the power storage device500. The aging can be performed under the following conditions, forexample. Charge is performed at a rate of 0.001 C or more and 0.2 C orless. The temperature may be higher than or equal to room temperatureand lower than or equal to 50° C. In the case where an electrolyte isdecomposed and a gas is generated and accumulates between theelectrodes, the electrolyte cannot be in contact with a surface of theelectrode in some regions. That is to say, an effectual reaction area ofthe electrode is reduced and effectual resistance is increased.

When the resistance is extremely increased, the negative electrodepotential is decreased. Consequently, lithium is intercalated intographite and lithium is deposited on the surface of graphite. Thelithium deposition might reduce capacity. For example, if a coating filmor the like is grown on the surface after lithium deposition, lithiumdeposited on the surface cannot be dissolved again. This lithium cannotcontribute to capacity. In addition, when deposited lithium isphysically collapsed and conduction with the electrode is lost, thelithium also cannot contribute to capacity. Therefore, the gas ispreferably released to prevent the potential of the negative electrodefrom reaching the potential of lithium because of an increase in acharging voltage.

In the case of performing degasification, for example, part of theexterior body of the thin power storage device is cut to open the powerstorage device. When the exterior body is expanded because of a gas, theform of the exterior body is preferably adjusted. Furthermore, theelectrolyte may be added as needed before resealing. In the case wheredegasification cannot be performed, a space for releasing a gas may beprovided in the cell so that a gas that accumulates between theelectrodes can be released from between the electrodes. A space formedby the use of the embossed laminate exterior body described above can beutilized as a space for releasing a gas.

After the release of the gas, the charging state may be maintained at atemperature higher than room temperature, preferably higher than orequal to 30° C. and lower than or equal to 60° C., more preferablyhigher than or equal to 35° C. and lower than or equal to 50° C. for,for example, 1 hour or more and 100 hours or less. In the initialcharging, an electrolyte decomposed on the surface forms a coating film.The formed coating film may thus be densified when the charging state isheld at a temperature higher than room temperature after the release ofthe gas, for example.

Here, a charging rate and a discharging rate will be described. Thecharging rate (also referred to as C rate) refers to the relative ratioof constant current charging current to battery capacity (current valuein charging [A]÷battery capacity [Ah]) and is expressed in a unit C. Forexample, the case where a battery having a capacity of 10 Ah is chargedat a constant current of 2 A is rephrased as follows: charging isperformed at 0.2 C. A charging rate of 1 C refers to the amount ofcurrent with which a battery is charged completely for one hour. Thehigher the charging rate is, the faster charging is completed.Furthermore, a discharging rate (also referred to as C rate) refers tothe relative ratio of constant current discharging current to batterycapacity (current value in discharging [A]÷battery capacity [Ah]) and isexpressed in a unit C. For example, the case where a battery having acapacity of 10 Ah is discharged at a constant current of 2 A isrephrased as follows: discharging is performed at 0.2 C. A dischargingrate of 1 C refers to the amount of current with which a battery isdischarged completely for one hour. The higher the discharging rate is,the faster discharging is completed.

<Components of Power Storage Device>

Other components of the power storage device of one embodiment of thepresent invention are described in detail below. When a flexiblematerial is selected from materials of the members described in thisembodiment and used, a flexible power storage device can be fabricated.

<<Electrolyte>>

The electrolyte contains a solute and a solvent.

As a solvent of the electrolyte, a material with carrier ion mobility isused. In particular, the solvent preferably has high heat resistance andlow reactivity to a graphite negative electrode. In the power storagedevice of one embodiment of the present invention, propylene carbonate,ethylene carbonate, and vinylene carbonate are mixed and used for thesolvent.

As the solvent, an aprotic organic solvent is preferably used. Forexample, propylene carbonate, ethylene carbonate, vinyl ethylenecarbonate, one of EC, PC, butylene carbonate, γ-butyrolactone,γ-valerolactone, dimethyl sulfoxide, methyl diglyme, benzonitrile, andsulfolane can be used, or two or more of these solvents can be used inan appropriate combination in an appropriate ratio.

When a gelled high-molecular material is used as the solvent of theelectrolyte, safety against liquid leakage and the like is improved.Further, the power storage device can be thinner and more lightweight.Typical examples of gelled high-molecular materials include a siliconegel, an acrylic gel, an acrylonitrile gel, a polyethylene oxide-basedgel, a polypropylene oxide-based gel, a gel of a fluorine-based polymer,and the like.

Alternatively, the use of one or more kinds of ionic liquids (roomtemperature molten salts) which have features of non-flammability andnon-volatility as a solvent of the electrolyte can prevent the powerstorage device from exploding or catching fire even when the powerstorage device internally shorts out or the internal temperatureincreases owing to overcharging and others. Thus, the power storagedevice has improved safety.

As the solute, a material that has carrier ion mobility and containscarrier ions can be used. In the case where carrier ions are lithiumions, the solute is a lithium salt. As a lithium salt, LiBETA, lithiumbis(trifluoromethanesulfonyl)amide (Li(CF₃SO₂)₂N, abbreviation: LiTFSA),lithium bis(fluorosulfonyl)amide (Li(FSO₂)₂N, abbreviation: LiFSA),LiBF₄, lithium bis(oxalate)borate (LiB(C₂O₄)₂, abbreviation: LiBOB), orthe like, which has high heat resistance, is preferably used.

In the power storage device, when a metal included in the positiveelectrode current collector is dissolved by a battery reaction betweenthe electrolyte and the current collector, the capacity of the powerstorage device is decreased and the power storage device deteriorates.That is, the capacity is significantly decreased as charging anddischarging are repeated through the cycle performance test of the powerstorage device, and the lifetime of the power storage device becomesshort. Furthermore, when metal dissolution from the current collector ata connection portion between the lead and the current collectorproceeds, disconnection might occur. In one embodiment of the presentinvention, a material which is unlikely to react with the currentcollector and thus is unlikely to cause the dissolution of the metal inthe current collector is used for the solute material contained in theelectrolyte.

Examples of a metal in materials for the positive electrode currentcollector include aluminum and stainless steel. In one embodiment of thepresent invention, for a solute material used for the electrolyte, thesolute that is unlikely to dissolve such a metal included in thepositive electrode current collector is used. Specifically, lithium saltrepresented by General Formula (G1) and LiPF₆ can be given as a lithiumsalt that can be used as the solute in one embodiment of the presentinvention.

In the power storage device of one embodiment of the present invention,the dissolution of the metal included in the positive electrode currentcollector into the electrolyte is inhibited, so that the deteriorationof the positive electrode current collector is inhibited. In addition,the deposition of the metal on a surface of the negative electrode isinhibited, so that the capacity reduction is small, and the powerstorage device can have a favorable cycle lifetime.

Other than the above solute, one of lithium salts such as LiPF₆, LiClO₄,LiAsF₆, LiAlCl₄, LiSCN, LiBr, LiI, Li₂SO₄, Li₂B₁₀Cl₁₀, Li₂B₁₂Cl₁₂,LiCF₃SO₃, LiC₄F₉SO₃, LiC(CF₃SO₂)₃, LiC(C₂F₅SO₂)₃, LiN(FSO₂)₂,LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, and LiN(C₄F₉SO₂) (CF₃SO₂) can be used, ortwo or more of these lithium salts can be used in an appropriatecombination in an appropriate ratio.

Although the case where carrier ions are lithium ions in the aboveelectrolyte is described, carrier ions other than lithium ions can beused. When the carrier ions other than lithium ions are alkali metalions or alkaline-earth metal ions, instead of lithium in the lithiumsalts, an alkali metal (e.g., sodium or potassium) or an alkaline-earthmetal (e.g., calcium, strontium, barium, beryllium, or magnesium) may beused as the solute.

Furthermore, an additive agent such as vinylene carbonate (VC), propanesultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC),lithium bis(oxalate)borate (LiBOB), or a dinitrile compound such assuccinonitrile or adiponitrile may be added to the electrolyte solution.The concentration of such an additive agent in the whole solvent is, forexample, higher than or equal to 0.1 wt % and lower than or equal to 5wt %.

With the use of the above solvent and the above electrolyte, anelectrolyte of the power storage device of one embodiment of the presentinvention can be formed.

<<Current Collector>>

There is no particular limitation on the current collector as long as ithas high conductivity without causing a significant chemical change in apower storage device. For example, the positive electrode currentcollector and the negative electrode current collector can each beformed using a metal such as stainless steel, gold, platinum, zinc,iron, nickel, copper, aluminum, titanium, tantalum, or manganese, analloy thereof, sintered carbon, or the like. Alternatively, copper orstainless steel that is coated with carbon, nickel, titanium, or thelike may be used. Alternatively, the current collectors can each beformed using an aluminum alloy to which an element that improves heatresistance, such as silicon, titanium, neodymium, scandium, ormolybdenum, is added. Still alternatively, a metal element that formssilicide by reacting with silicon can be used to form the currentcollectors. Examples of the metal element that forms silicide byreacting with silicon include zirconium, titanium, hafnium, vanadium,niobium, tantalum, chromium, molybdenum, tungsten, cobalt, and nickel.

An irreversible reaction with an electrolyte is sometimes caused onsurfaces of the positive electrode current collector and the negativeelectrode current collector. Thus, the positive electrode currentcollector and the negative electrode current collector preferably havelow reactivity to an electrolyte.

The positive electrode current collector and the negative electrodecurrent collector can each have any of various shapes including afoil-like shape, a plate-like shape (sheet-like shape), a net-likeshape, a cylindrical shape, a coil shape, a punching-metal shape, anexpanded-metal shape, a porous shape, and a shape of non-woven fabric asappropriate. The positive electrode current collector and the negativeelectrode current collector may each be formed to have microirregularities on the surface thereof in order to enhance adhesion tothe active material layer. The positive electrode current collector andthe negative electrode current collector each preferably have athickness of 5 μm to 30 μm inclusive.

An undercoat layer may be provided over part of a surface of the currentcollector. The undercoat layer is a coating layer provided to reducecontact resistance between the current collector and the active materiallayer or to improve adhesion between the current collector and theactive material layer. Note that the undercoat layer is not necessarilyformed over the entire surface of the current collector and may bepartly formed to have an island-like shape. In addition, the undercoatlayer may serve as an active material to have capacity. For theundercoat layer, a carbon material can be used, for example. Examples ofthe carbon material include graphite, carbon black such as acetyleneblack, and a carbon nanotube. Examples of the undercoat layer include ametal layer, a layer containing carbon and high molecular compounds, anda layer containing metal and high molecular compounds.

<<Active Material Layer>>

The active material layer includes the active material. An activematerial refers only to a material that is involved in insertion andextraction of ions that are carriers. In this specification and thelike, a layer including an active material is referred to as an activematerial layer. The active material layer may include a conductiveadditive and a binder in addition to the active material.

The positive electrode active material layer includes one or more kindsof positive electrode active materials. The negative electrode activematerial layer includes one or more kinds of negative electrode activematerials.

The positive electrode active material and the negative electrode activematerial have a central role in battery reactions of a power storagedevice, and receive and release carrier ions. To increase the lifetimeof the power storage device, the active materials preferably have alittle capacity involved in irreversible battery reactions, and havehigh charge and discharge efficiency.

Examples of positive electrode active materials are a composite oxidewith a layered rock-salt crystal structure and a composite oxide with aspinel crystal structure. Alternatively, an example of a positiveelectrode active material is a polyanionic positive electrode material.Examples of polyanionic positive electrode materials are a material withan olivine crystal structure and a material with a NASICON structure.Alternatively, an example of a positive electrode active material is apositive electrode material containing sulfur.

As the positive electrode active material, various composite oxides canbe used. For the active material particles, a compound such as LiFeO₂,LiCoO₂, LiNiO₂, LiMn₂O₄, Li₂MnO₃, V₂O₅, Cr₂O₅, or MnO₂ can be used.

An example of a material with a layered rock-salt crystal structureincludes a composite oxide represented by LiMO₂. The element M ispreferably one or more elements selected from Co and Ni. LiCoO₂ ispreferable because it has high capacity, stability in the air, andthermal stability to a certain extent, for example. As the element M,one or more elements selected from Al and Mn may be included in additionto one or more elements selected from Co and Ni.

For example, it is possible to use LiNi_(x)Mn_(y)Co_(z)O_(w) (x, y, andz are each ⅓ or a neighborhood thereof and w is 2 or a neighborhoodthereof, for example). For example, it is possible to useLiNi_(x)Mn_(y)Co_(z)O_(w) (x is 0.8 or a neighborhood thereof, y is 0.1or a neighborhood thereof, z is 0.1 or a neighborhood thereof, and w is2 or a neighborhood thereof, for example). For example, it is possibleto use LiNi_(x)Mn_(y)Co_(z)O_(w) (x is 0.5 or a neighborhood thereof, yis 0.3 or a neighborhood thereof, z is 0.2 or a neighborhood thereof,and w is 2 or a neighborhood thereof, for example). For example, it ispossible to use LiNi_(x)Mn_(y)Co_(z)O_(w) (x is 0.6 or a neighborhoodthereof, y is 0.2 or a neighborhood thereof, z is 0.2 or a neighborhoodthereof, and w is 2 or a neighborhood thereof, for example). Forexample, it is possible to use LiNi_(x)Mn_(y)Co_(z)O_(w) (x is 0.4 or aneighborhood thereof, y is 0.4 or a neighborhood thereof, z is 0.2 or aneighborhood thereof, and w is 2 or a neighborhood thereof, forexample).

A neighborhood is a value greater than 0.9 times and smaller than 1.1times the predetermined value.

A material in which part of the transition metal and lithium included inthe positive electrode active material is replaced with one or moreelements selected from Fe, Co, Ni, Cr, Al, Mg, and the like, or amaterial in which the positive electrode active material is doped withone or more elements selected from Fe, Co, Ni, Cr, Al, Mg, and the likemay be used for the positive electrode active material.

As the positive electrode active material, for example, a solid solutionobtained by combining two or more composite oxides can be used. Forexample, a solid solution of LiNi_(x)Mn_(y)Co_(z)O₂ (x, y, z>0, x+y+z=1)and Li₂MnO₃ can be used as the positive electrode active material.

An example of a material with a spinel crystal structure includes acomposite oxide represented by LiM₂O₄. It is preferable to contain Mn asthe element M. For example, LiMn₂O₄ can be used. It is preferable tocontain Ni in addition to Mn as the element M because the dischargevoltage and the energy density of the secondary battery are improved insome cases. It is preferable to add a small amount of lithium nickeloxide (LiNiO₂ or LiNi_(1-x)M_(x)O₂ (M=Co, Al, or the like)) to alithium-containing material with a spinel crystal structure whichcontains manganese, such as LiMn₂O₄, because the characteristics of thesecondary battery can be improved.

The average diameter of primary particles of the positive electrodeactive material is preferably greater than or equal to 1 nm and lessthan or equal to 100 μm, further preferably greater than or equal to 50nm and less than or equal to 50 μm, and still further preferably greaterthan or equal to 1 μm and less than or equal to 30 μm, for example.Furthermore, the specific surface area is preferably greater than orequal to 1 m²/g and less than or equal to 20 m²/g. Furthermore, theaverage diameter of secondary particles is preferably greater than orequal to 5 μm and less than or equal to 50 μm. Note that the averageparticle diameters can be measured with a particle diameter distributionanalyzer or the like using a laser diffraction and scattering method orby observation with a scanning electron microscope (SEM) or a TEM. Thespecific surface area can be measured by a gas adsorption method.

A conductive material such as a carbon layer may be provided on thesurface of the positive electrode active material. With the conductivematerial such as the carbon layer, the conductivity of the electrode canbe increased. For example, the positive electrode active material can becoated with a carbon layer by mixing a carbohydrate such as glucose atthe time of baking the positive electrode active material. As theconductive material, graphene, multi-graphene, graphene oxide (GO), orreduced graphene oxide (RGO) can be used. Note that RGO refers to acompound obtained by reducing graphene oxide (GO), for example.

Note that a layer containing an oxide and/or a fluoride may be providedon a surface of the positive electrode active material. The oxide andthe positive electrode active material may differ or similar incomposition.

As the polyanionic positive electrode material, for example, a complexoxide containing oxygen, an element X, a metal A, and a metal M. Themetal M is one or more elements selected from Fe, Mn, Co, Ni, Ti, V, andNb, the metal A is one or more elements selected from Li, Na, and Mg,and the element X is one or more elements selected from S, P, Mo, W, As,and Si.

As a material with an olivine crystal structure, a complex material(LiMPO₄ (general formula) (M is one or more of Fe(II), Mn(II), Co(II),and Ni(II))) can be used as the positive electrode active material.Typical examples of the general formula LiMPO₄ are lithium compoundssuch as LiFePO₄, LiNiPO₄, LiCoPO₄, LiMnPO₄, LiFe_(a)Ni_(b)PO₄,LiFe_(a)Co_(b)PO₄, LiFe_(a)Mn_(b)PO₄, LiNi_(a)Co_(b)PO₄,LiNi_(a)Mn_(b)PO₄ (a+b<1, 0<a<1, and 0<b<1), LiFe_(c)Ni_(d)Co_(e)PO₄,LiFe_(c)Ni_(d)Mn_(e)PO₄, LiNi_(c)Co_(d)Mn_(e)PO₄ (c+d+e<1, 0<c<1, 0<d<1,and 0<e<1), and LiFe_(f)Ni_(g)Co_(h)Mn_(i)PO₄ (f+g+h+i≤1, 0<f<1, 0<g<1,0<h<1, and 0<i<1).

In particular, LiFePO₄ is preferable because it meets requirements withbalance for the positive electrode active material, such as safety,stability, high capacity density, high potential, and the existence oflithium ions that can be extracted in initial oxidation (charging).

The average diameter of primary particles of the positive electrodeactive material with an olivine crystal structure is preferably greaterthan or equal to 1 nm and less than or equal to 20 μm, furtherpreferably greater than or equal to 10 nm and less than or equal to 5μm, and still further preferably greater than or equal to 50 nm and lessthan or equal to 2 μm, for example. Furthermore, the specific surfacearea is preferably greater than or equal to 1 m²/g and less than orequal to 20 m²/g. Furthermore, the average diameter of secondaryparticles is preferably greater than or equal to 5 μm and less than orequal to 50 μm.

Alternatively, a complex material such as Li_((2-j))MSiO₄ (generalformula) (M is one or more of Fe(II), Mn(II), Co(II), and Ni(II); 0≤j≤2)may be used as the positive electrode active material. Typical examplesof the general formula Li_((2-j))MSiO₄ are lithium compounds such asLi_((2-j))FeSiO₄, Li_((2-j))CoSiO₄, Li_((2-j))MnSiO₄,Li_((2-j))Fe_(k)Ni_(l)SiO₄, Li_((2-j))Fe_(k)CoSiO₄,Li_((2-j))Fe_(k)Mn_(l)SiO₄, Li_((2-j))Ni_(k)Co_(l)SiO₄,Li_((2-j))Ni_(k)Mn_(l)SiO₄ (k+l<1, 0<k<1, and 0<l<1),Li_((2-j))Fe_(m)Ni_(n)Co_(g)SiO₄, Li_((2-j))Fe_(m)Ni_(n)Mn_(g)SiO₄,Li_((2-j))Ni_(m)Co_(n)Mn_(g)SiO₄ (m+n+q≤1, 0<m<1, 0<n<1, and 0<q<1), andLi_((2-j))Fe_(r)Ni_(s)Co_(t)Mn_(u)SiO₄ (r+s+t+u≤1, 0<r<1, 0<s<1, 0<t<1,and 0<u<1).

Still alternatively, a nasicon compound represented by A_(x)M₂(XO₄)₃(general formula) (A=Li, Na, or Mg, M=Fe, Mn, Ti, V, or Nb, X═S, P, Mo,W, As, or Si) can be used for the positive electrode active material.Examples of the nasicon compound are Fe₂(MnO₄)₃, Fe₂(SO₄)₃, andLi₃Fe₂(PO₄)₃. Further alternatively, a compound represented by Li₂MPO₄F,Li₂MP₂O₇, or Li₅MO₄ (general formula) (M=Fe or Mn) can be used as thepositive electrode active material.

Further alternatively, a polyanionic positive electrode materialcontaining V can be used. Typical examples are α-LiVOPO₄, β-LiVOPO₄,α1-LiVOPO₄, LiVPO₄F, LiVPO₄O, LiVP₂O₇, LiVOSO₄, Li₂VOSiO₄, and LiVMoO₆.

Further alternatively, a perovskite fluoride such as NaFeF₃ and FeF₃, ametal chalcogenide (a sulfide, a selenide, or a telluride) such as TiS₂and MoS₂, an oxide with an inverse spinel structure such as LiMVO₄, avanadium oxide (V₂O₅, V₆O₁₃, LiV₃O₈, or the like), a manganese oxide, anorganic sulfur compound, or the like can be used as the positiveelectrode active material.

Alternatively, a borate-based positive electrode material represented byLiMBO₃ (general formula) (M is Fe(II), Mn(II), or Co(II)) can be used asthe positive electrode active material.

Another example of the positive electrode active material is alithium-manganese composite oxide represented by a composition formulaLi_(a)Mn_(b)M_(c)O_(d). Here, the element M is preferably a metalelement other than lithium and manganese, or silicon or phosphorus,further preferably nickel. Furthermore, in the case where the wholeparticle of a lithium-manganese composite oxide is measured, it ispreferable to satisfy the following at the time of discharging:0<a/(b+c)<2; c>0; and 0.26≤(b+c)/d<0.5. To achieve a high capacity, thelithium-manganese composite oxide preferably includes a region where thesurface portion and the middle portion are different in the crystalstructure, the crystal orientation, or the oxygen content. In order thatsuch a lithium-manganese composite oxide can be obtained, thecomposition formula is preferably 1.6≤a≤1.848; 0.19≤c/b≤0.935; and2.5≤d≤3. Furthermore, it is particularly preferable to use alithium-manganese composite oxide represented by a composition formulaLi_(1.68)Mn_(0.8062)Ni_(0.318)O₃. In this specification and the like, alithium-manganese composite oxide represented by a composition formulaLi_(1.68)Mn_(0.8062)Ni_(0.318)O₃ refers to that formed at a ratio (molarratio) of the amounts of raw materials ofLi₂CO₃:MnCO₃:Ni₀=0.84:0.8062:0.318. Although this lithium-manganesecomposite oxide is represented by a composition formulaLi_(1.68)Mn_(0.8062)Ni_(0.318)O₃, the composition might deviate fromthis.

Note that the ratios of metal, silicon, phosphorus, and other elementsto the total composition in the whole particle of a lithium-manganesecomposite oxide can be measured with, for example, an inductivelycoupled plasma mass spectrometer (ICP-MS). The ratio of oxygen to thetotal composition in the whole particle of a lithium-manganese compositeoxide can be measured by, for example, energy dispersive X-rayspectroscopy (EDX). Alternatively, the ratio of oxygen to the totalcomposition in the whole particle of a lithium-manganese composite oxidecan be measured by ICP-MS combined with fusion gas analysis and valenceevaluation of X-ray absorption fine structure (XAFS) analysis. Note thatthe lithium-manganese composite oxide is an oxide containing at leastlithium and manganese, and may contain at least one selected fromchromium, cobalt, aluminum, nickel, iron, magnesium, molybdenum, zinc,indium, gallium, copper, titanium, niobium, silicon, phosphorus, and thelike.

In the case where carrier ions are alkali metal ions other than lithiumions, or alkaline-earth metal ions, a material containing an alkalimetal (e.g., sodium or potassium) or an alkaline-earth metal (e.g.,calcium, strontium, barium, beryllium, or magnesium) instead of lithiummay be used as the positive electrode active material. For example, thepositive electrode active material may be a layered oxide containingsodium.

As the positive electrode active material, for example, an oxidecontaining sodium, such as NaFeO₂, Na_(2/3)[Fe_(1/2)Mn_(1/2)]O₂,Na_(2/3)[Ni_(1/3)Mn_(2/3)]O₂, Na₂Fe₂(SO₄)₃, Na₃V₂(PO₄)₃, Na₂FePO₄F,NaVPO₄F, NaMPO₄ (M is Fe(II), Mn(II), Co(II), or Ni(II)), Na₂FePO₄F, orNa₄Co₃(PO₄)₂P₂O₇ can be used.

In addition, as the positive electrode active material, alithium-containing metal sulfide can be used. Examples of thelithium-containing metal sulfide are Li₂TiS₃ and Li₃NbS₄.

As the negative electrode active material, for example, a carbon-basedmaterial, an alloy-based material, or the like can be used.

Examples of the carbon-based material include graphite, graphitizingcarbon (soft carbon), non-graphitizing carbon (hard carbon), a carbonnanotube, graphene, carbon black, and the like. Examples of the graphiteinclude artificial graphite such as meso-carbon microbeads (MCMB),coke-based artificial graphite, or pitch-based artificial graphite andnatural graphite such as spherical natural graphite. In addition,examples of the shape of the graphite include a flaky shape and aspherical shape.

Graphite has a low potential substantially equal to that of a lithiummetal when lithium ions are intercalated into the graphite (while alithium-graphite intercalation compound is formed). For this reason, alithium-ion secondary battery can have a high operating voltage.Graphite is preferred because of its advantages described above, such asrelatively high capacity per unit volume, small volume expansion, lowcost, and safety greater than that of a lithium metal.

Here, a graphite material will be described. Graphite is a layeredcompound in which a plurality of graphene layers are stacked parallel toeach other by van der Waals forces. A surface of the graphite materialincludes a plane parallel to the graphene layer (also referred to as abasal plane) and a plane where edges of the plurality of graphene layersare arranged (also referred to as an edge plane). In the basal plane,one surface of the outmost layer of the graphene layers composinggraphite is exposed. In the edge plane, the edges of the plurality ofgraphene layers are exposed. In charging and discharging of a secondarybattery, the edge plane of the graphite material serves as a main gatefor lithium intercalation and deintercalation to and from the graphitematerial.

In the case where graphite is used for the negative electrode activematerial, the contact between the exposed portion of the edge plane andthe electrolyte containing PC might cause a side reaction betweengraphite and PC in charging and discharging. In spherical naturalgraphite used for the negative electrode active material included in thepower storage device of one embodiment of the present invention, a layerhaving lower crystallinity than a graphite layer is formed in contactwith the edge plane; thus, a side reaction between graphite and PC canbe inhibited in some cases.

For example, in the case where carrier ions are lithium ions, a materialincluding at least one of Mg, Ca, Ga, Si, Al, Ge, Sn, Pb, As, Sb, Bi,Ag, Au, Zn, Cd, Hg, In, and the like can be used as the alloy-basedmaterial. Such elements have a higher capacity than carbon. Inparticular, silicon has a high theoretical capacity of 4200 mAh/g, andtherefore, the capacity of the power storage device can be increased.Examples of an alloy-based material (compound-based material) using suchelements include Mg₂Si, Mg₂Ge, Mg₂Sn, SnS₂, V₂Sn₃, FeSn₂, CoSn₂, Ni₃Sn₂,Cu₆Sn₅, Ag₃Sn, Ag₃Sb, Ni₂MnSb, CeSb₃, LaSn₃, La₃Co₂Sn₇, CoSb₃, InSb, andSbSn.

Alternatively, for the negative electrode active material, an oxide suchas SiO, SnO, SnO₂, titanium dioxide (TiO₂), lithium titanium oxide(Li₄Ti₅O₁₂), lithium-graphite intercalation compound (LixC₆), niobiumpentoxide (Nb₂O₅), tungsten oxide (WO₂), or molybdenum oxide (MoO₂) canbe used. Here, SiO is a compound containing silicon and oxygen. When theatomic ratio of silicon to oxygen is represented by α:β, a preferablyhas an approximate value of β. Here, when a has an approximate value ofβ, an absolute value of the difference between α and β is preferablyless than or equal to 20% of a value of β, more preferably less than orequal to 10% of a value of β.

Still alternatively, for the negative electrode active material,Li_(3-x)M_(x)N (M=Co, Ni, or Cu) with a Li₃N structure, which is anitride containing lithium and a transition metal, can be used. Forexample, Li_(2.6)Co_(0.4)N₃ is preferable because of high charge anddischarge capacity (900 mAh/g and 1890 mAh/cm³).

When a nitride containing lithium and a transition metal is used,lithium ions are contained in the negative electrode active material andthus the negative electrode active material can be used in combinationwith a material for a positive electrode active material that does notcontain lithium ions, such as V₂O₅ or Cr₃O₈. In the case of using amaterial containing lithium ions as a positive electrode activematerial, the nitride containing lithium and a transition metal can beused for the negative electrode active material by extracting thelithium ions contained in the positive electrode active material inadvance.

Alternatively, a material that causes a conversion reaction can be usedfor the negative electrode active material; for example, a transitionmetal oxide that does not cause an alloy reaction with lithium, such ascobalt oxide (CoO), nickel oxide (NiO), and iron oxide (FeO), may beused. Other examples of the material which causes a conversion reactioninclude oxides such as Fe₂O₃, CuO, Cu₂O, RuO₂, and Cr₂O₃, sulfides suchas CoS_(0.89), NiS, and CuS, nitrides such as Zn₃N₂, Cu₃N, and Ge₃N₄,phosphides such as NiP₂, FeP₂, and CoP₃, and fluorides such as FeF₃ andBiF₃.

The average diameter of primary particles of the negative electrodeactive material is preferably, for example, greater than or equal to 5nm and less than or equal to 100 μm.

The positive electrode active material layer and the negative electrodeactive material layer may each include a conductive additive.

Examples of the conductive additive include a carbon material, a metalmaterial, and a conductive ceramic material. Alternatively, a fibermaterial may be used as the conductive additive. The content of theconductive additive in the active material layer is preferably greaterthan or equal to 1 wt % and less than or equal to 10 wt %, morepreferably greater than or equal to 1 wt % and less than or equal to 5wt %.

A network for electric conduction can be formed in the electrode by theconductive additive. The conductive additive also allows maintaining ofa path for electric conduction between the negative electrode activematerial particles. The addition of the conductive additive to theactive material layer increases the electric conductivity of the activematerial layer.

Examples of the conductive additive include natural graphite, artificialgraphite such as mesocarbon microbeads, and carbon fiber. Examples ofcarbon fiber include mesophase pitch-based carbon fiber, isotropicpitch-based carbon fiber, carbon nanofiber, and carbon nanotube. Carbonnanotube can be formed by, for example, a vapor deposition method. Otherexamples of the conductive additive include carbon materials such ascarbon black (e.g., acetylene black (AB)), graphite (black lead)particles, graphene, graphene oxide, and fullerene. Alternatively, metalpowder or metal fibers of copper, nickel, aluminum, silver, gold, or thelike, a conductive ceramic material, or the like can be used.

Flaky graphene has an excellent electrical characteristic of highconductivity and excellent physical properties of high flexibility andhigh mechanical strength. Thus, the use of graphene as the conductiveadditive can increase contact points and the contact area of the activematerials.

Graphene is capable of making low-resistance surface contact and hasextremely high conductivity even with a small thickness. Therefore, evena small amount of graphene can efficiently form a conductive path in anactive material layer.

In the case where an active material with a small average particlediameter (e.g., 1 μm or less) is used, the specific surface area of theactive material is large and thus more conductive paths for the activematerial particles are needed. In such a case, it is particularlypreferred that graphene with extremely high conductivity that canefficiently form a conductive path even in a small amount is used.

The positive electrode active material layer and the negative electrodeactive material layer may each include a binder.

In this specification, the binder has a function of binding or bondingthe active materials and/or a function of binding or bonding the activematerial layer and the current collector. The binder is sometimeschanged in state during fabrication of an electrode or a battery. Forexample, the binder can be at least one of a liquid, a solid, and a gel.The binder is sometimes changed from a monomer to a polymer duringfabrication of an electrode or a battery.

As the binder, for example, a water-soluble high molecular compound canbe used. As the water-soluble high molecular compound, a polysaccharideor the like can be used. As the polysaccharide, a cellulose derivativesuch as carboxymethyl cellulose (CMC), methyl cellulose, ethylcellulose, hydroxypropyl cellulose, diacetyl cellulose, or regeneratedcellulose, starch, or the like can be used.

As the binder, a rubber material such as styrene-butadiene rubber (SBR),styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber,butadiene rubber, fluororubber, or ethylene-propylene-diene copolymercan be used. Any of these rubber materials may be used in combinationwith the aforementioned water-soluble high molecular compound. Sincethese rubber materials have rubber elasticity and easily expand andcontract, it is possible to obtain a highly reliable electrode that isresistant to stress due to expansion and contraction of an activematerial by charging and discharging, bending of the electrode, or thelike. On the other hand, the rubber materials have a hydrophobic groupand thus are unlikely to be soluble in water in some cases. In such acase, particles are dispersed in an aqueous solution without beingdissolved in water, so that increasing the viscosity of a compositioncontaining a solvent used for the formation of the active material layer(also referred to as an electrode mix composition) up to the viscositysuitable for application might be difficult. A water-soluble highmolecular compound having excellent viscosity modifying properties, suchas a polysaccharide, can moderately increase the viscosity of thesolution and can be uniformly dispersed together with a rubber material.Thus, a favorable electrode with high uniformity (e.g., an electrodewith uniform electrode thickness or electrode resistance) can beobtained.

Alternatively, as the binder, a material such as PVDF, polystyrene,poly(methyl acrylate), poly(methyl methacrylate) (PMMA), sodiumpolyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO),polypropylene oxide, polyimide, polyvinyl chloride,polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene,polyethylene terephthalate, nylon, polyacrylonitrile (PAN),ethylene-propylene-diene polymer, polyvinyl acetate, or nitrocellulosecan be used.

Two or more of the above materials may be used in combination for thebinder.

The content of the binder in the active material layer is preferablygreater than or equal to 1 wt % and less than or equal to 10 wt %, morepreferably greater than or equal to 2 wt % and less than or equal to 8wt %, and still more preferably greater than or equal to 2 wt % and lessthan or equal to 5 wt %.

<<Separator>>

In view of the above, in the power storage device of one embodiment ofthe present invention, a separator containing polyphenylene sulfide(PPS) or a separator containing solvent-spun regenerated cellulosicfiber is used. The separator may have a single-layer structure or astacked-layer structure. A layered structure of a separator containingsolvent-spun regenerated cellulosic fiber and another separator may beused, for example.

As a material for the separator, one or more materials selected from thefollowing can be used besides polyphenylene sulfide and cellulosicfiber: polypropylene sulfide, a fluorine-based polymer, cellulose,paper, nonwoven fabric, glass fiber, ceramics, synthetic fiber such asnylon (polyamide), vinylon (polyvinyl alcohol fiber), polyester,acrylic, polyolefin, or polyurethane, and the like.

<<Exterior Body>>

It is preferred that the surface of the exterior body 509 that is incontact with the electrolyte 508, i.e., the inner surface of theexterior body 509, does not react with the electrolyte 508significantly. When moisture enters the power storage device 500 fromthe outside, a reaction between a component of the electrolyte 508 orthe like and water might occur. Thus, the exterior body 509 preferablyhas low moisture permeability.

As the exterior body 509, a film having a three-layer structure can beused, for example. In the three-layer structure, a highly flexible metalthin film of aluminum, stainless steel, copper, nickel, or the like isprovided over a film formed using polyethylene, polypropylene,polycarbonate, ionomer, polyamide, or the like, and an insulatingsynthetic resin film of a polyamide-based resin, a polyester-basedresin, or the like is provided as the outer surface of the exterior bodyover the metal thin film can be used. With such a three-layer structure,the passage of an electrolyte and a gas can be blocked and an insulatingproperty and resistance to the electrolyte can be provided. The exteriorbody is folded inside in two, or two exterior bodies are stacked withthe inner surfaces facing each other, in which case application of heatmelts the materials on the overlapping inner surfaces to cause fusionbonding between the two exterior bodies. In this manner, a sealingstructure can be formed.

A portion where the sealing structure is formed by fusion bonding or thelike of the exterior body is referred to as a sealing portion. In thecase where the exterior body is folded inside in two, the sealingportion is formed in the place other than the fold, and a first regionof the exterior body and a second region of the exterior body thatoverlaps with the first region are fusion-bonded, for example. In thecase where two exterior bodies are stacked, the sealing portion isformed along the entire outer region by heat fusion bonding or the like.

The power storage device 500 can be flexible by using the exterior body509 with flexibility. When the power storage device 500 has flexibility,it can be used in an electronic device at least part of which isflexible, and the power storage device 500 can be bent as the electronicdevice changes its form.

Note that in one embodiment of the present invention, a graphenecompound can be used in a component of the power storage device. Asdescribed later, when modification is performed, the structure andcharacteristics of a graphene compound can be selected from a widerrange of alternatives. Thus, a preferable property can be exhibited inaccordance with a component in which a graphene compound is to be used.Moreover, a graphene compound has high mechanical strength and thereforecan be used in a component of a flexible power storage device. Graphenecompounds will be described below.

Graphene has carbon atoms arranged in one atomic layer. A π bond existsbetween the carbon atoms. Graphene including two or more and one hundredor less layers is referred to as multilayer graphene in some cases. Thelength in the longitudinal direction or the length of the major axis ina plane in each of graphene and multilayer graphene is greater than orequal to 50 nm and less than or equal to 100 μm or greater than or equalto 800 nm and less than or equal to 50 μm.

In this specification and the like, a compound including graphene ormultilayer graphene as a basic skeleton is referred to as a graphenecompound. Graphene compounds include graphene and multilayer graphene.

Graphene compounds will be detailed below.

A graphene compound is, for example, a compound where graphene ormultilayer graphene is modified with an atom other than carbon or anatomic group with an atom other than carbon. A graphene compound may bea compound where graphene or multilayer graphene is modified with anatomic group composed mainly of carbon, such as an alkyl group oralkylene. An atomic group that modifies graphene or multilayer grapheneis referred to as a substituent, a functional group, a characteristicgroup, or the like in some cases. Modification in this specification andthe like refers to introduction of an atom other than carbon, an atomicgroup with an atom other than carbon, or an atomic group composed mainlyof carbon to graphene, multilayer graphene, a graphene compound, orgraphene oxide (described later) by a substitution reaction, an additionreaction, or other reactions.

Note that the surface and the rear surface of graphene may be modifiedwith different atoms or atomic groups. In multilayer graphene, multiplelayers may be modified with different atoms or atomic groups.

An example of the above-described graphene modified with an atom or anatomic group is graphene or multilayer graphene that is modified withoxygen or a functional group containing oxygen. Examples of a functionalgroup containing oxygen include an epoxy group, a carbonyl group such asa carboxyl group, and a hydroxyl group. A graphene compound modifiedwith oxygen or a functional group containing oxygen is referred to asgraphene oxide in some cases. In this specification, graphene oxidesinclude multilayer graphene oxides.

As an example of modification of graphene oxide, silylation of grapheneoxide will be described. First, in a nitrogen atmosphere, graphene oxideis put in a container, n-butylamine (C₄H₉NH₂) is added to the container,and stirring is performed for one hour with the temperature kept at 60°C. Then, toluene is added to the container, alkyltrichlorosilane isadded thereto as a silylating agent, and stirring is performed in anitrogen atmosphere for five hours with the temperature kept at 60° C.Then, toluene is further added to the container, and the resultingsolution is suction-filtrated to give solid powder. The powder isdispersed in ethanol, and the resulting solution is suction-filtered togive solid powder. The powder is dispersed in acetone, and the resultingsolution is suction-filtered to give solid powder. A liquid of the solidpowder is vaporized to give silylated graphene oxide.

The modification is not limited to silylation, and silylation is notlimited to the above-described method. Furthermore, the modification isnot limited to introduction of an atom or an atomic group of one kind,and the modification of two or more types may be performed to introduceatoms or atomic groups of two or more kinds. By introducing a givenatomic group to a graphene compound, the physical property of thegraphene compound can be changed. Therefore, by performing desirablemodification in accordance with the use of a graphene compound, adesired property of the graphene compound can be exhibitedintentionally.

A formation method example of graphene oxide will be described below.Graphene oxide can be obtained by oxidizing the aforementioned grapheneor multilayer graphene. Alternatively, graphene oxide can be obtained bybeing separated from graphite oxide. Graphite oxide can be obtained byoxidizing graphite. The graphene oxide may further be modified with theabove-mentioned atom or atomic group.

A compound that can be obtained by reducing graphene oxide is referredto as reduced graphene oxide (RGO) in some cases. In RGO, in some cases,all oxygen atoms contained in the graphene oxide are not extracted andpart of them remains in a state of oxygen or an atomic group containingoxygen that is bonded to carbon. In some cases, RGO includes afunctional group, e.g., an epoxy group, a carbonyl group such as acarboxyl group, or a hydroxyl group.

A graphene compound may have a sheet-like shape where a plurality ofgraphene compounds partly overlap with each other. Such a graphenecompound is referred to as a graphene compound sheet in some cases. Thegraphene compound sheet has, for example, an area with a thicknesslarger than or equal to 0.33 nm and smaller than or equal to 10 mm,preferably larger than or equal to 0.34 nm and smaller than or equal to10 μm. The graphene compound sheet may be modified with an atom otherthan carbon, an atomic group containing an atom other than carbon, anatomic group composed mainly of carbon such as an alkyl group, or thelike. A plurality of layers in the graphene compound sheet may bemodified with different atoms or atomic groups.

A graphene compound may have a five-membered ring composed of carbonatoms or a poly-membered ring that is a seven- or more-membered ringcomposed of carbon atoms, in addition to a six-membered ring composed ofcarbon atoms. In the neighborhood of a poly-membered ring which is aseven- or more-membered ring, a region through which a lithium ion canpass may be generated.

Furthermore, a plurality of graphene compounds may be gathered to form asheet-like shape, for example.

A graphene compound has a planar shape, thereby enabling surfacecontact.

In some cases, a graphene compound has high conductivity even when it isthin. The contact area between graphene compounds or between a graphenecompound and an active material can be increased by surface contact.Thus, even with a small amount of a graphene compound per volume, aconductive path can be formed efficiently.

In contrast, a graphene compound may also be used as an insulator. Forexample, a graphene compound sheet can be used as a sheet-likeinsulator. Graphene oxide, for example, has a more excellent insulationproperty than a graphene compound that is not oxidized, in some cases. Agraphene compound modified with an atomic group may have an improvedinsulation property, depending on the type of the modifying atomicgroup.

A graphene compound in this specification and the like may include aprecursor of graphene. The precursor of graphene refers to a substanceused to form graphene. The precursor of graphene may contain theabove-described graphene oxide, graphite oxide, or the like.

Graphene containing an alkali metal or an element other than carbon,such as oxygen, is referred to as a graphene analog in some cases. Inthis specification and the like, graphene compounds include grapheneanalogs.

A graphene compound in this specification and the like may include anatom, an atomic group, and ions of them between the layers. The physicalproperties, such as electric conductivity and ionic conductivity, of agraphene compound sometimes change when an atom, an atomic group, andions of them exist between layers of the compound. In addition, adistance between the layers is increased in some cases.

A graphene compound has excellent electrical characteristics of highconductivity and excellent physical properties of high flexibility andhigh mechanical strength in some cases. A modified graphene compound canhave extremely low conductivity and serve as an insulator depending onthe type of the modification. A graphene compound has a planar shape. AGraphene compound enables low-resistance surface contact.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 2

In this embodiment, electronic devices of embodiments of the presentinvention will be described with reference to FIG. 14A to 14C to FIGS.17A to 17C.

<Structural Example 1 of Smartwatch>

FIG. 14A is a perspective view of a watch-type portable informationterminal (also called a smartwatch) 700. The portable informationterminal 700 includes a housing 701, a display panel 702, a clasp 703,bands 705A and 705B, and operation buttons 711 and 712.

The display panel 702 mounted in the housing 701 doubling as a bezelincludes a rectangular display region. The display region has a curvedsurface. The display panel 702 preferably has flexibility. Note that thedisplay region may be non-rectangular.

The bands 705A and 705B are connected to the housing 701. The clasp 703is connected to the band 705A. The band 705A and the housing 701 areconnected to each other with a pin such that they can pivot around thepin at a connection portion, for example. In a similar manner, the band705B and the housing 701 are connected to each other and the band 705Aand the clasp 703 are connected to each other.

FIGS. 14B and 14C are perspective views of the band 705A and a powerstorage device 750, respectively. The band 705A includes the powerstorage device 750. As the power storage device 750, the power storagedevice 500 described in Embodiment 1 can be used, for example. The powerstorage device 750 is embedded in the band 705A, and part of thepositive electrode lead 751 and part of the negative electrode lead 752protrude from the band 705A (see FIG. 14B). The positive electrode lead751 and the negative electrode lead 752 are electrically connected tothe display panel 702. The surface of the power storage device 750 iscovered with an exterior body 753 (see FIG. 14C). Note that the pin mayfunction as an electrode. Specifically, the positive electrode lead 751and the display panel 702 may be electrically connected to each otherthrough the pin that connects the band 705A and the housing 701, and thenegative electrode lead 752 and the display panel 702 may beelectrically connected to each other through the pin. In that case, thestructure of the connection portion between the band 705A and thehousing 701 can be simplified.

The power storage device 750 has flexibility. Specifically, a surface ofthe exterior body 753 preferably has projections and depressions formedby the embossing described in Embodiment 1. Furthermore, the powerstorage device 750 preferably has sliding planes of the power storagedevice 500 illustrated in FIGS. 5A and 5B.

The band 705A can be formed so as to incorporate the power storagedevice 750. For example, the power storage device 750 is set in a moldthat the outside shape of the band 705A fits and a material of the band705A is poured in the mold and cured, so that the band 705A illustratedin FIG. 14B can be formed.

In the case where a rubber material is used as the material of the band705A, rubber is cured through heat treatment. For example, in the casewhere fluorine rubber is used as a rubber material, it is cured throughheat treatment at 170° C. for 10 minutes. In the case where siliconerubber is used as a rubber material, it is cured through heat treatmentat 150° C. for 10 minutes. The power storage device of one embodiment ofthe present invention has high heat resistance, which can inhibitbreakage and degradation of the charge and discharge characteristics dueto heat treatment performed when the power storage device and the rubbermaterial are integrally formed.

Examples of the material of the band 705A include fluorine rubber,silicone rubber, fluorosilicone rubber, and urethane rubber.

Note that energization of the power storage device 750, including aging,is preferably performed after the power storage device 750 is formed tobe incorporated in the band 705A. In other words, heat treatment ispreferably performed on the power storage device 500 described inEmbodiment 1 before energization of the power storage device 500. Theheat treatment is preferably performed at 110° C. to 190° C. inclusivefor a period of time suitable for vulcanization of the rubber material,for example, at 170° C. for 10 minutes. This can inhibit degradation ofthe charge and discharge characteristics of the power storage device 500due to heat treatment.

The portable information terminal 700 in FIG. 14A can have a variety offunctions such as a function of displaying a variety of data (e.g., astill image, a moving image, and a text image) on the display region, atouch panel function, a function of displaying a calendar, date, time,and the like, a function of controlling processing with a variety ofsoftware (programs), a wireless communication function, a function ofbeing connected to a variety of computer networks with a wirelesscommunication function, a function of transmitting and receiving avariety of data with a wireless communication function, and a functionof reading out a program or data stored in a recording medium anddisplaying it on the display region.

The housing 701 can include a speaker, a sensor (a sensor having afunction of measuring force, displacement, position, speed,acceleration, angular velocity, rotational frequency, distance, light,liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radiation,flow rate, humidity, gradient, oscillation, odor, or infrared rays), amicrophone, and the like. Note that the portable information terminal700 can be manufactured using the light-emitting element for the displaypanel 702.

Although FIGS. 14A to 14C illustrate the example where the power storagedevice 750 is incorporated in the band 705A, the power storage device750 may be incorporated in the band 705B. The band 705B can be formedusing a material similar to that of the band 705A.

The rubber material used for the band 705A preferably has high chemicalresistance. Specifically, the rubber material preferably has lowreactivity to an electrolyte contained in the power storage device 750.

When the band 705A is cracked or chipped despite of its high chemicalresistance, a user of the portable information terminal 700 might touchthe electrolyte that leaks from the power storage device 750. In thecase where the portable information terminal 700 has a function ofdetecting leakage of the electrolyte, the user can stop operation of theportable information terminal 700 and remove it as soon as theelectrolyte leakage is detected. Consequently, the portable informationterminal 700 can be highly safe.

<Structural Example 2 of Smartwatch>

FIG. 15A is a perspective view of a band 735A having a structuredifferent form that of the band 705A illustrated in FIG. 14B. A housing731 connected to the band 735A includes a leakage detection circuit (notillustrated) having a function of detecting leakage of the electrolyteof the power storage device (see FIG. 14A). Note that a perspective viewof a portable information terminal 730 including the leakage detectioncircuit is similar to that of the portable information terminal 700.

The band 735A includes a power storage device 760. The power storagedevice 760 is embedded in the band 735A, and part of the positiveelectrode lead 751, part of the negative electrode lead 752, part of aterminal 761, and part of a terminal 762 protrude from the band 735A.The positive electrode lead 751 and the negative electrode lead 752 areelectrically connected to the display panel 702. The terminal 761 andthe terminal 762 are electrically connected to the above leakagedetection circuit, for example.

FIG. 15B is a perspective view of the power storage device 760. FIG. 15Bis an enlarged view of FIG. 15A for clarification. The power storagedevice 760 is different from the power storage device 750 in FIG. 14C inincluding the terminal 761, terminal 762, a wiring 771, and a wiring772. The terminal 761 is electrically connected to the wiring 771. Theterminal 762 is electrically connected to the wiring 772.

Although the wiring 771 and the wiring 772 are indicated by differenthatch patterns for clarification in FIG. 15B, the wiring 771 and thewiring 772 are preferably formed using the same material to achieve costreduction. Although the terminal 761 and the wiring 771 are indicated bythe same hatching pattern and the terminal 762 and the wiring 772 areindicated by the same hatching pattern, the terminal 761 and the wiring771 may be formed using different materials and the terminal 762 and thewiring 772 may be formed using different materials.

The wiring 771 and the wiring 772 are provided on a surface of theexterior body 753 with a predetermined gap therebetween (see FIG. 15B).If the electrolyte leaks to a surface of the exterior body 753, thewiring 771 and the wiring 772 are electrically connected to each otherthrough the electrolyte, whereby the leakage detection circuit candetect leakage of the electrolyte.

Although the wiring 771 and the wiring 772 each having a linear shapeare provided in the direction of the long axis of the power storagedevice 760 in FIG. 15B, one embodiment of the present invention is notlimited thereto. For example, as illustrated in FIG. 15C, the wiring 771and the wiring 772 each having a comb-like shape may be provided with agap therebetween so as to engage with each other.

Although FIG. 15C illustrates the example where the wirings 771 and 772are provided only on the top surface of the exterior body 753, thewirings 771 and 772 are preferably provided on the entire surface of theexterior body 753 as illustrated in FIG. 16A. FIG. 16B is a perspectiveview of the back surface of the power storage device 760 illustrated inFIG. 16A.

The wirings 771 and 772 each preferably have a small thickness and asmall width, in which case the flexibility of the power storage device760 can be ensured. For example, the power storage device 760 preferablyincludes a region in which the thickness of each of the wirings 771 and772 is 5 μm to 500 μm inclusive. Furthermore, it is preferred that thegap between the wiring 771 and the wiring 772 be small and the width ofeach of the wiring 771 and the wiring 772 be small, in which case even asmall amount of electrolyte leakage can be detected. For example, thepower storage device 760 preferably includes a region in which thelength of the gap between the wirings 771 and 772 is 0.5 mm and 20 mminclusive. Furthermore, the power storage device 760 preferably includesa region in which the width of each of the wiring 771 and the wiring 772is 0.5 mm and 5 mm inclusive. When the occupation area of the wirings771 and 772 in the surface of the exterior body 753 is excessivelysmall, the detection of electrolyte leakage for the entire surface ofthe exterior body 753 is not possible in some cases, whereas when theoccupation area is excessively large, the flexibility of the powerstorage device 760 is low in some cases. In the power storage device760, the proportion of the surface area of the wirings 771 and 772except side surfaces thereof (the surfaces in contact with the exteriorbody 753) to the surface area of the exterior body 753 is preferably 5%to 50% inclusive.

The wirings 771 and 772 preferably include a material having highductility or high malleability. In particular, the use of a materialhaving both high ductility and high malleability can suppress breakageof the wirings 771 and 772 due to curving of the power storage device760. Examples of the material having both high ductility and highmalleability include a metal material such as gold, silver, platinum,iron, nickel, copper, aluminum, zinc, and tin and an alloy containingany of the metal materials.

<<Leakage Detecting Method>>

An example of a method for detecting electrolyte leakage in the portableinformation terminal 730 will be described below. FIG. 17A is a blockdiagram of the configuration of the portable information terminal 730when the electrolyte 736 leaks. In FIG. 17A, lines with arrows indicatethe directions in which a wired signal or a wireless signal istransmitted. Thus, components connected by the corresponding line areelectrically connected to each other in some cases. Lines without anarrow indicate wirings, and components connected by the correspondingline are electrically connected to each other.

The portable information terminal 730 includes a leakage detectioncircuit 732, a power source 733, an ammeter 734, the wiring 771, and thewiring 772 (see FIG. 17A). The leakage detection circuit 732, the powersource 733, and the ammeter 734 are included in the housing 731. Thepower source 733 and the ammeter 734 may be included in the leakagedetection circuit 732. The portable information terminal 730 alsoincludes a functional circuit 739. The functional circuit 739 includesthe speaker, the sensor, the microphone, and the like. The functionalcircuit 739 is included in the housing 731.

The wirings 771 and 772 are electrically connected to the power source733, and a given voltage is applied between the wiring 771 and thewiring 772 (see FIG. 17A). The on/off of the power source 733 iscontrolled by the leakage detection circuit 732.

FIG. 17B is a flow chart showing the flow of detection of electrolyteleakage in the portable information terminal 730. The method fordetecting electrolyte leakage in the portable information terminal 730includes the following four steps, for example.

When the electrolyte 736 of the power storage device 760 leaks, theelectrolyte 736 is attached to a surface of the exterior body 753 (seeFIG. 17A and Si in FIG. 17B). The electrolyte 736 attached to thesurface of the exterior body 753 comes in contact with the wiring 771and the wiring 772, whereby a current flows through the wiring 771 andthe wiring 772 (see S2 in FIG. 17B). On detecting the current, theammeter 734 connected to be in parallel to the wiring 772 outputs adetection signal to the leakage detection circuit 732 (see S3 in FIG.17B). The leakage detection circuit 732 terminates the operation of thedisplay panel 702 and/or the functional circuit 739 in response to thedetection signal (see S4 in FIG. 17B).

Although FIG. 17A illustrates the example where the ammeter 734 isconnected to the wiring 772, the ammeter 734 may be connected to thewiring 771. Furthermore, the power source 733 and the ammeter 734 may beincluded in the leakage detection circuit 732, and the leakage detectioncircuit 732 may be electrically connected to the wirings 771 and 772(see FIG. 17C). In that case, the leakage detection circuit 732 has afunction of applying a predetermined voltage to the wirings 771 and 772and a function of detecting a current flowing through the wirings 771and 772.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 3

In this embodiment, flexible power storage devices that are embodimentsof the present invention will be described with reference to FIGS. 18Aand 18B to FIG. 25. The power storage device of one embodiment of thepresent invention may have a curved shape. The power storage device ofone embodiment of the present invention may be flexible and capable ofbeing used while being curved and while being not curved.

<Structural Example 1>

FIG. 18A is a perspective view of a secondary battery 200 and FIG. 18Bis a top view of the secondary battery 200.

FIG. 19A is a cross-sectional view along dashed-dotted line C₁-C₂ inFIG. 18B, and FIG. 19B is a cross-sectional view along dashed-dottedline C₃-C₄ in FIG. 18B. Note that FIGS. 19A and 19B do not illustrateall components for clarity of the drawings.

The secondary battery 200 includes a positive electrode 211, a negativeelectrode 215, and a separator 203. The secondary battery 200 furtherincludes a positive electrode lead 221, a negative electrode lead 225,and an exterior body 207.

The positive electrode 211 and the negative electrode 215 each include acurrent collector and an active material layer. The positive electrode211 and the negative electrode 215 are provided such that the activematerial layers face each other with the separator 203 providedtherebetween.

One of the electrodes (the positive electrode 211 and the negativeelectrode 215) of the secondary battery 200 that is positioned on theouter diameter side of a curved portion is preferably longer than theother electrode that is positioned on the inner diameter side of thecurved portion, in the direction in which the electrode is curved. Withsuch a structure, ends of the positive electrode 211 and those of thenegative electrode 215 are aligned when the secondary battery 200 iscurved with a certain curvature. That is, the entire region of thepositive electrode active material layer included in the positiveelectrode 211 can face the negative electrode active material layerincluded in the negative electrode 215. Thus, positive electrode activematerials included in the positive electrode 211 can efficientlycontribute to a battery reaction. Therefore, the capacity of thesecondary battery 200 per volume can be increased. Such a structure isparticularly effective in a case where the curvature of the secondarybattery 200 is fixed in using the secondary battery 200.

The positive electrode lead 221 is electrically connected to a pluralityof positive electrodes 211. The negative electrode lead 225 iselectrically connected to a plurality of negative electrodes 215. Thepositive electrode lead 221 and the negative electrode lead 225 eachinclude a sealing layer 220.

The exterior body 207 covers a plurality of positive electrodes 211, aplurality of negative electrodes 215, and a plurality of separators 203.The secondary battery 200 includes an electrolyte (not shown) in aregion covered with the exterior body 207. Three sides of the exteriorbody 207 are bonded, whereby the secondary battery 200 is sealed.

In FIGS. 21A and 21B, the separators 203 each having a strip-like shapeare used and each pair of the positive electrode 211 and the negativeelectrode 215 sandwich the separator 203; however, one embodiment of thepresent invention is not limited to this structure. One separator sheetmay be folded in zigzag (or into a bellows shape) or wound so that theseparator is positioned between the positive electrode and the negativeelectrode.

An example of a method for fabricating the secondary battery 200 isillustrated in FIGS. 21A to 21D. FIG. 20 is a cross-sectional view alongdashed-dotted line C1-C2 in FIG. 18B of the case of employing thismanufacturing method.

First, the negative electrode 215 is positioned over the separator 203(FIG. 21A) such that the negative electrode active material layer of thenegative electrode 215 overlaps with the separator 203.

Then, the separator 203 is folded to overlap with the negative electrode215. Next, the positive electrode 211 overlaps with the separator 203(FIG. 21B) such that the positive electrode active material layer of thepositive electrode 211 overlaps with the separator 203 and the negativeelectrode active material layer. Note that in the case of using anelectrode in which one surface of a current collector is provided withan active material layer, the positive electrode active material layerof the positive electrode 211 and the negative electrode active materiallayer of the negative electrode 215 are positioned to face each otherwith the separator 203 provided therebetween.

In the case where the separator 203 is formed using a material that canbe thermally welded, such as polypropylene, a region where the separator203 overlaps with itself is thermally welded and then another electrodeoverlaps with the separator 203, whereby the slippage of the electrodein the fabrication process can be suppressed. Specifically, a regionwhich does not overlap with the negative electrode 215 or the positiveelectrode 211 and in which the separator 203 overlaps with itself, e.g.,a region denoted as 203 a in FIG. 21B, is preferably thermally welded.

By repeating the above steps, the positive electrode 211 and thenegative electrode 215 can overlap with each other with the separator203 provided therebetween as illustrated in FIG. 21C.

Note that a plurality of positive electrodes 211 and a plurality ofnegative electrodes 215 may be placed to be alternately sandwiched bythe separator 203 that is repeatedly folded in advance.

Then, as illustrated in FIG. 21C, a plurality of positive electrodes 211and a plurality of negative electrodes 215 are covered with theseparator 203.

Furthermore, the region where the separator 203 overlaps with itself,e.g., a region 203 b in FIG. 21D, is thermally welded as illustrated inFIG. 21D, whereby a plurality of positive electrodes 211 and a pluralityof negative electrodes 215 are covered with and tied with the separator203.

Note that a plurality of positive electrodes 211, a plurality ofnegative electrodes 215, and the separator 203 may be tied with abinding material.

Since the positive electrodes 211 and the negative electrodes 215 arestacked in the above process, one separator 203 has a region sandwichedbetween a plurality of positive electrodes 211 and a plurality ofnegative electrodes 215 and a region covering a plurality of positiveelectrodes 211 and a plurality of negative electrodes 215.

In other words, the separator 203 included in the secondary battery 200in FIG. 20 and FIG. 21D is a single separator which is partly folded. Inthe folded regions of the separator 203, a plurality of positiveelectrodes 211 and a plurality of negative electrodes 215 are provided.

<Structural Example 2>

FIG. 22A is a perspective view of a secondary battery 250 and FIG. 22Bis a top view of the secondary battery 250. Furthermore, FIG. 22C1 is across-sectional view of a first electrode assembly 230 and FIG. 22C2 isa cross-sectional view of a second electrode assembly 231.

The secondary battery 250 includes the first electrode assembly 230, thesecond electrode assembly 231, and the separator 203. The secondarybattery 250 further includes the positive electrode lead 221, thenegative electrode lead 225, and the exterior body 207.

As illustrated in FIG. 22C1, in the first electrode assembly 230, apositive electrode 211 a, the separator 203, a negative electrode 215 a,the separator 203, and the positive electrode 211 a are stacked in thisorder. The positive electrode 211 a and the negative electrode 215 aeach include active material layers on opposite surfaces of a currentcollector.

As illustrated in FIG. 22C2, in the second electrode assembly 231, anegative electrode 215 a, the separator 203, the positive electrode 211a, the separator 203, and the negative electrode 215 a are stacked inthis order. The positive electrode 211 a and the negative electrode 215a each include active material layers on opposite surfaces of a currentcollector.

In other words, in each of the first electrode assembly 230 and thesecond electrode assembly 231, the positive electrode and the negativeelectrode are provided such that the active material layers face eachother with the separator 203 provided therebetween.

The positive electrode lead 221 is electrically connected to a pluralityof positive electrodes 211. The negative electrode lead 225 iselectrically connected to a plurality of negative electrodes 215. Thepositive electrode lead 221 and the negative electrode lead 225 eachinclude the sealing layer 220.

FIG. 23 is an example of a cross-sectional view along dashed-dotted lineD1-D2 in FIG. 22B. Note that FIG. 23 does not illustrate all componentsfor clarity of the drawings.

As illustrated in FIG. 23, the secondary battery 250 has a structure inwhich a plurality of first electrode assemblies 230 and a plurality ofsecond electrode assemblies 231 are covered with the wound separator203.

The exterior body 207 covers a plurality of first electrode assemblies230, a plurality of second electrode assemblies 231, and the separator203. The secondary battery 200 includes an electrolyte (not shown) in aregion covered with the exterior body 207. Three sides of the exteriorbody 207 are bonded, whereby the secondary battery 200 is sealed.

An example of a method for fabricating the secondary battery 250 isillustrated in FIGS. 24A to 24D.

First, the first electrode assembly 230 is positioned over the separator203 (FIG. 24A).

Then, the separator 203 is folded to overlap with the first electrodeassembly 230. After that, two second electrode assemblies 231 arepositioned over and under the first electrode assembly 230 with theseparator 203 positioned between each of the second electrode assemblies231 and the first electrode assembly 230 (FIG. 24B).

Then, the separator 203 is wound to cover the two second electrodeassemblies 231. Moreover, two first electrode assemblies 230 arepositioned over and under the two second electrode assemblies 231 withthe separator 203 positioned between each of the first electrodeassemblies 230 and each of the second electrode assemblies 231 (FIG.24C).

Then, the separator 203 is wound to cover the two first electrodeassemblies 230 (FIG. 24D).

Since a plurality of first electrode assemblies 230 and a plurality ofsecond electrode assemblies 231 are stacked in the above process, theseelectrode assemblies are each positioned surrounded with the spirallywound separator 203.

Note that the outermost electrode preferably does not include an activematerial layer on the outer side.

Although FIGS. 22C1 and 22C2 each illustrate a structure in which theelectrode assembly includes three electrodes and two separators, oneembodiment of the present invention is not limited to this structure.The electrode assembly may include four or more electrodes and three ormore separators. A larger number of electrodes lead to higher capacityof the secondary battery 250. Alternatively, the electrode assembly mayinclude two electrodes and one separator. A smaller number of electrodesenable higher resistance of the secondary battery against bending.Although FIG. 23 illustrates the structure in which the secondarybattery 250 includes three first electrode assemblies 230 and two secondelectrode assemblies 231, one embodiment of the present invention is notlimited to this structure. The number of the electrode assemblies may beincreased. A larger number of electrode assemblies lead to highercapacity of the secondary battery 250. The number of the electrodeassemblies may be decreased. A smaller number of electrode assembliesenable higher resistance of the secondary battery against bending.

FIG. 23 illustrates another example of a cross-sectional view alongdashed-dotted line D1-D2 in FIG. 22B. As illustrated in FIG. 23, theseparator 203 may be folded into a bellows shape so that the separator203 is positioned between the first electrode assembly 230 and thesecond electrode assembly 231.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 4

In this embodiment, application examples of the power storage device ofone embodiment of the present invention will be described with referenceto FIGS. 26A to 26F to FIGS. 30A and 30B.

The power storage device of one embodiment of the present invention canbe used for an electronic device or a lighting device, for example. Thepower storage device of one embodiment of the present invention hasexcellent charge and discharge characteristics. Therefore, theelectronic device or the lighting device can be used for a long time bya single charge. Moreover, since a decrease in capacity with anincreasing number of charge and discharge cycles is inhibited, the timebetween charges is unlikely to be reduced by repetitive charge.Furthermore, the power storage device of one embodiment of the presentinvention exhibits excellent charge and discharge characteristics andhigh long-term reliability and is highly safe at a wide range oftemperature including high temperatures, so that the safety andreliability of an electronic device or a lighting device can beimproved.

Examples of electronic devices include a television set (also referredto as a television or a television receiver), a monitor of a computer orthe like, a digital camera, a digital video camera, a digital photoframe, a mobile phone (also referred to as a mobile phone device), aportable game machine, a portable information terminal, an audioreproducing device, a large game machine such as a pinball machine, andthe like.

Since the power storage device of one embodiment of the presentinvention is flexible, the power storage device or an electronic deviceor a lighting device using the power storage device can be incorporatedalong a curved inside/outside wall surface of a house or a building or acurved interior/exterior surface of a motor vehicle.

FIG. 26A illustrates an example of a mobile phone. A mobile phone 7400is provided with a display portion 7402 incorporated in a housing 7401,an operation button 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone 7400includes a power storage device 7407.

FIG. 26B illustrates the mobile phone 7400 in the state of being bent.When the whole mobile phone 7400 is bent by the external force, thepower storage device 7407 included in the mobile phone 7400 is alsobent. The power storage device 7407 is a thin power storage device. Thepower storage device 7407 is fixed in a state of being bent. FIG. 26Cillustrates the power storage device 7407 in the state of being bent

FIG. 26D illustrates an example of a bangle display device. A portabledisplay device 7100 includes a housing 7101, a display portion 7102, anoperation button 7103, and a power storage device 7104. FIG. 26Eillustrates the bent power storage device 7104.

FIG. 26F illustrates an example of a watch-type portable informationterminal. A portable information terminal 7200 includes a housing 7201,a display portion 7202, a band 7203, a buckle 7204, an operation button7205, an input output terminal 7206, and the like.

The portable information terminal 7200 is capable of executing a varietyof applications such as mobile phone calls, e-mailing, viewing andediting text, music reproduction, Internet communication, and a computergame.

The display surface of the display portion 7202 is curved, and imagescan be displayed on the curved display surface. In addition, the displayportion 7202 includes a touch sensor, and operation can be performed bytouching the screen with a finger, a stylus, or the like. For example,by touching an icon 7207 displayed on the display portion 7202,application can be started.

With the operation button 7205, a variety of functions such as timesetting, power ON/OFF, ON/OFF of wireless communication, setting andcancellation of silent mode, and setting and cancellation of powersaving mode can be performed. For example, the functions of theoperation button 7205 can be set freely by the operating systemincorporated in the portable information terminal 7200.

Furthermore, the portable information terminal 7200 can employ nearfield communication, which is a communication method based on anexisting communication standard. In that case, for example, mutualcommunication between the portable information terminal 7200 and aheadset capable of wireless communication can be performed, and thushands-free calling is possible.

Moreover, the portable information terminal 7200 includes the inputoutput terminal 7206, and data can be directly transmitted to andreceived from another information terminal via a connector. In addition,charging via the input output terminal 7206 is possible. Note that thecharging operation may be performed by wireless power feeding withoutusing the input output terminal 7206.

The display portion 7202 of the portable information terminal 7200 isprovided with the power storage device of one embodiment of the presentinvention. For example, the power storage device 7104 illustrated inFIG. 26E that is in the state of being curved can be provided in thehousing 7201. Alternatively, the power storage device 7104 illustratedin FIG. 26E can be provided in the band 7203 such that it can be curved.

FIG. 27A illustrates an example of a wrist-worn activity meter. Theactivity meter 7250 includes a housing 7251, the band 7203, the buckle7204, and the like. Furthermore, the housing 7251 incorporates awireless communication device, a pulse sensor, an acceleration sensor, atemperature sensor, and the like. The activity meter 7250 has a functionof acquiring data such as pulse variation and the amount of activity ofthe user with the pulse sensor and the acceleration sensor and sendingthe data to an external portable information terminal by the wirelesscommunication device. Furthermore, the activity meter 7250 may have afunction of measuring calorie consumption and calorie intake of theuser, a function of measuring the number of steps taken, a function ofmeasuring a sleeping condition, or the like. Note that the activitymeter 7250 may be provided with a display portion for displaying dataacquired by the above function.

The activity meter 7250 includes the power storage device of oneembodiment of the present invention. For example, the power storagedevice 7104 illustrated in FIG. 26E that is in the state of being curvedcan be provided in the housing 7201. Alternatively, the power storagedevice 7104 illustrated in FIG. 26E can be provided in the band 7203such that it can be curved.

FIG. 27B illustrates an example of an armband display device. A displaydevice 7300 includes a display portion 7304 and the power storage deviceof one embodiment of the present invention. The display device 7300 caninclude a touch sensor in the display portion 7304 and can serve as aportable information terminal.

The display surface of the display portion 7304 is bent, and images canbe displayed on the bent display surface. A display state of the displaydevice 7300 can be changed by, for example, near field communication,which is a communication method based on an existing communicationstandard.

The display device 7300 includes an input output terminal, and data canbe directly transmitted to and received from another informationterminal via a connector. In addition, charging via the input outputterminal is possible. Note that the charging operation may be performedby wireless power feeding without using the input output terminal.

FIG. 27C illustrates an example of a glasses-type display device. Adisplay device 7350 includes lenses 7351, a frame 7352, and the like.Furthermore, a projection portion (not illustrated) that projects animage or video on the lenses 7351 is provided in the frame 7352 or incontact with the frame 7352. The display device 7350 has a function ofdisplaying an image 7351A on the entire lenses 7351 in the direction inwhich the user can see the image 7351A. Alternatively, the displaydevice 7350 has a function of displaying an image 7351B on part of thelenses 7351 in the direction in which the user can see the image 7351B.

The display device 7350 includes the power storage device of oneembodiment of the present invention. FIG. 27D is an enlarged view of anedge portion 7355 of the frame 7352. The edge portion 7355 can be formedusing a rubber material such as fluorine rubber or silicone rubber. Thepower storage device 7360 of one embodiment of the present invention isembedded in the edge portion 7355, and the positive electrode lead 7361and the negative electrode lead 7362 protrude from the edge portion7355. The positive electrode lead 7361 and the negative electrode lead7362 are electrically connected to a wiring provided in the frame 7352and connected to a projection portion or the like. Note that the edgeportion 7355 can be formed so as to incorporate the power storage device7360 as in Embodiment 2.

The edge portion 7355 and the power storage device 7360 haveflexibility. Thus, the display device 7350 can be worn so as to be inclose contact with the user along with the shape of the head of theuser.

FIGS. 28A and 28B illustrate an example of a tablet terminal that can befolded in half A tablet terminal 9600 illustrated in FIGS. 28A and 28Bincludes a pair of housings 9630, a movable portion 9640 connecting thepair of housings 9630, a display portion 9631 a, a display portion 9631b, a display mode changing switch 9626, a power switch 9627, a powersaving mode changing switch 9625, a fastener 9629, and an operationswitch 9628. FIG. 28A illustrates the tablet terminal 9600 that isopened, and FIG. 28B illustrates the tablet terminal 9600 that isclosed.

The tablet terminal 9600 includes a power storage unit 9635 inside thehousings 9630. The power storage unit 9635 is provided across thehousings 9630, passing through the movable portion 9640.

Part of the display portion 9631 a can be a touch panel region 9632 a,and data can be input by touching operation keys 9638 that aredisplayed. Note that FIG. 28A shows, as an example, that half of thearea of the display portion 9631 a has only a display function and theother half of the area has a touch panel function. However, thestructure of the display portion 9631 a is not limited to this, and allthe area of the display portion 9631 a may have a touch panel function.For example, all the area of the display portion 9631 a can display akeyboard and serve as a touch panel while the display portion 9631 b canbe used as a display screen.

As in the display portion 9631 a, part of the display portion 9631 b canbe a touch panel region 9632 b. When a keyboard display switching button9639 displayed on the touch panel is touched with a finger, a stylus, orthe like, a keyboard can be displayed on the display portion 9631 b.

Touch input can be performed in the touch panel region 9632 a and thetouch panel region 9632 b at the same time.

The display mode changing switch 9626 allows switching between alandscape mode and a portrait mode, color display and black-and-whitedisplay, and the like. The power saving mode changing switch 9625 cancontrol display luminance in accordance with the amount of externallight in use of the tablet terminal 9600, which is measured with anoptical sensor incorporated in the tablet terminal 9600. In addition tothe optical sensor, other detecting devices such as sensors fordetermining inclination, such as a gyroscope or an acceleration sensor,may be incorporated in the tablet terminal.

Although the display portion 9631 a and the display portion 9631 b havethe same area in FIG. 28A, one embodiment of the present invention isnot limited to this example. The display portion 9631 a and the displayportion 9631 b may have different areas or different display quality.For example, one of the display portions 9631 a and 9631 b may displayhigher definition images than the other.

The tablet terminal is closed in FIG. 28B. The tablet terminal includesthe housings 9630, a solar cell 9633, and a charge and discharge controlcircuit 9634 including a DCDC converter 9636. The power storage deviceof one embodiment of the present invention is used as the power storageunit 9635.

The tablet terminal 9600 can be folded such that the housings 9630overlap with each other when not in use. Thus, the display portions 9631a and 9631 b can be protected, which increases the durability of thetablet terminal 9600. In addition, the power storage unit 9635 of oneembodiment of the present invention has flexibility and can berepeatedly bent without a significant decrease in charge and dischargecapacity. Thus, a highly reliable tablet terminal can be provided.

The tablet terminal illustrated in FIGS. 28A and 28B can also have afunction of displaying various kinds of data (e.g., a still image, amoving image, and a text image) on the display portion, a function ofdisplaying a calendar, a date, or the time on the display portion, atouch-input function of operating or editing data displayed on thedisplay portion by touch input, a function of controlling processing byvarious kinds of software (programs), and the like.

The solar cell 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch panel, a display portion,an image signal processing portion, and the like. Note that the solarcell 9633 can be provided on one surface or opposite surfaces of thehousing 9630 and the power storage unit 9635 can be charged efficiently.The use of a lithium-ion battery as the power storage unit 9635 bringsan advantage such as reduction in size.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 28B will be described with reference to a blockdiagram in FIG. 28C. The solar cell 9633, the power storage unit 9635,the DCDC converter 9636, a converter 9637, switches SW1 to SW3, and thedisplay portion 9631 are illustrated in FIG. 28C, and the power storageunit 9635, the DCDC converter 9636, the converter 9637, and the switchesSW1 to SW3 correspond to the charge and discharge control circuit 9634in FIG. 28B.

First, an example of operation when electric power is generated by thesolar cell 9633 using external light will be described. The voltage ofelectric power generated by the solar cell is raised or lowered by theDCDC converter 9636 to a voltage for charging the power storage unit9635. When the display portion 9631 is operated with the electric powerfrom the solar cell 9633, the switch SW1 is turned on and the voltage ofthe electric power is raised or lowered by the converter 9637 to avoltage needed for operating the display portion 9631. When display onthe display portion 9631 is not performed, the switch SW1 is turned offand the switch SW2 is turned on, so that the power storage unit 9635 canbe charged.

Note that the solar cell 9633 is described as an example of a powergeneration means; however, one embodiment of the present invention isnot limited to this example. The power storage unit 9635 may be chargedusing another power generation means such as a piezoelectric element ora thermoelectric conversion element (Peltier element). For example, thepower storage unit 9635 may be charged with a non-contact powertransmission module capable of performing charging by transmitting andreceiving electric power wirelessly (without contact), or any of theother charge means used in combination.

FIG. 29 illustrates other examples of electronic devices. In FIG. 29, adisplay device 8000 is an example of an electronic device including apower storage device 8004 of one embodiment of the present invention.Specifically, the display device 8000 corresponds to a display devicefor TV broadcast reception and includes a housing 8001, a displayportion 8002, speaker portions 8003, and the power storage device 8004.The power storage device 8004 of one embodiment of the present inventionis provided in the housing 8001. The display device 8000 can receiveelectric power from a commercial power supply. Alternatively, thedisplay device 8000 can use electric power stored in the power storagedevice 8004. Thus, the display device 8000 can be operated with the useof the power storage device 8004 of one embodiment of the presentinvention as an uninterruptible power supply even when electric powercannot be supplied from a commercial power supply due to power failureor the like.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoresis displaydevice, a digital micromirror device (DMD), a plasma display panel(PDP), or a field emission display (FED) can be used for the displayportion 8002.

Note that the display device includes, in its category, all ofinformation display devices for personal computers, advertisementdisplays, and the like besides TV broadcast reception.

In FIG. 29, an installation lighting device 8100 is an example of anelectronic device including a power storage device 8103 of oneembodiment of the present invention. Specifically, the lighting device8100 includes a housing 8101, a light source 8102, and the power storagedevice 8103. Although FIG. 29 illustrates the case where the powerstorage device 8103 is provided in a ceiling 8104 on which the housing8101 and the light source 8102 are installed, the power storage device8103 may be provided in the housing 8101. The lighting device 8100 canreceive electric power from a commercial power supply. Alternatively,the lighting device 8100 can use electric power stored in the powerstorage device 8103. Thus, the lighting device 8100 can be operated withthe use of power storage device 8103 of one embodiment of the presentinvention as an uninterruptible power supply even when electric powercannot be supplied from a commercial power supply due to power failureor the like.

Note that although the installation lighting device 8100 provided in theceiling 8104 is illustrated in FIG. 29 as an example, the power storagedevice of one embodiment of the present invention can be used in aninstallation lighting device provided in, for example, a wall 8105, afloor 8106, a window 8107, or the like other than the ceiling 8104.Alternatively, the power storage device of one embodiment of the presentinvention can be used in a tabletop lighting device or the like.

As the light source 8102, an artificial light source which emits lightartificially by using electric power can be used. Specifically, anincandescent lamp, a discharge lamp such as a fluorescent lamp, andlight-emitting elements such as an LED and an organic EL element aregiven as examples of the artificial light source.

In FIG. 29, an air conditioner including an indoor unit 8200 and anoutdoor unit 8204 is an example of an electronic device including apower storage device 8203 of one embodiment of the present invention.Specifically, the indoor unit 8200 includes a housing 8201, an airoutlet 8202, and the power storage device 8203. Although FIG. 29illustrates the case where the power storage device 8203 is provided inthe indoor unit 8200, the power storage device 8203 may be provided inthe outdoor unit 8204. Alternatively, the power storage devices 8203 maybe provided in both the indoor unit 8200 and the outdoor unit 8204. Theair conditioner can receive electric power from a commercial powersupply. Alternatively, the air conditioner can use electric power storedin the power storage device 8203. Particularly in the case where thepower storage devices 8203 are provided in both the indoor unit 8200 andthe outdoor unit 8204, the air conditioner can be operated with the useof the power storage device 8203 of one embodiment of the presentinvention as an uninterruptible power supply even when electric powercannot be supplied from a commercial power supply due to power failureor the like.

Note that although the split-type air conditioner including the indoorunit and the outdoor unit is illustrated in FIG. 29 as an example, thepower storage device of one embodiment of the present invention can beused in an air conditioner in which the functions of an indoor unit andan outdoor unit are integrated in one housing.

In FIG. 29, an electric refrigerator-freezer 8300 is an example of anelectronic device including a power storage device 8304 of oneembodiment of the present invention. Specifically, the electricrefrigerator-freezer 8300 includes a housing 8301, a door for arefrigerator 8302, a door for a freezer 8303, and the power storagedevice 8304. The power storage device 8304 is provided in the housing8301 in FIG. 29. The electric refrigerator-freezer 8300 can receiveelectric power from a commercial power supply. Alternatively, theelectric refrigerator-freezer 8300 can use electric power stored in thepower storage device 8304. Thus, the electric refrigerator-freezer 8300can be operated with the use of the power storage device 8304 of oneembodiment of the present invention as an uninterruptible power supplyeven when electric power cannot be supplied from a commercial powersupply due to power failure or the like.

Note that a high-frequency heating apparatus such as a microwave ovenand an electronic device such as an electric rice cooker require highpower in a short time. The tripping of a breaker of a commercial powersupply in use of an electronic device can be prevented by using thepower storage device of one embodiment of the present invention as anauxiliary power supply for supplying electric power which cannot besupplied enough by a commercial power supply.

In addition, in a time period when electronic devices are not used,particularly when the proportion of the amount of electric power whichis actually used to the total amount of electric power which can besupplied from a commercial power supply source (such a proportionreferred to as a usage rate of electric power) is low, electric powercan be stored in the power storage device, whereby the usage rate ofelectric power can be reduced in a time period when the electronicdevices are used. For example, in the case of the electricrefrigerator-freezer 8300, electric power can be stored in the powerstorage device 8304 in night time when the temperature is low and thedoor for a refrigerator 8302 and the door for a freezer 8303 are notoften opened or closed. On the other hand, in daytime when thetemperature is high and the door for a refrigerator 8302 and the doorfor a freezer 8303 are frequently opened and closed, the power storagedevice 8304 is used as an auxiliary power supply; thus, the usage rateof electric power in daytime can be reduced.

Furthermore, the power storage device of one embodiment of the presentinvention can be provided in a vehicle.

The use of power storage devices in vehicles enables production ofnext-generation clean energy vehicles such as hybrid electric vehicles(HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles(PHEVs).

FIGS. 30A and 30B each illustrate an example of a vehicle using oneembodiment of the present invention. An automobile 8400 illustrated inFIG. 30A is an electric vehicle that runs on the power of an electricmotor. Alternatively, the automobile 8400 is a hybrid electric vehiclecapable of driving appropriately using either the electric motor or theengine. One embodiment of the present invention can provide ahigh-mileage vehicle. The automobile 8400 includes the power storagedevice. The power storage device is used not only for driving theelectric motor, but also for supplying electric power to alight-emitting device such as a headlight 8401 or a room light (notillustrated).

The power storage device can also supply electric power to a displaydevice of a speedometer, a tachometer, or the like included in theautomobile 8400. Furthermore, the power storage device can supplyelectric power to a semiconductor device included in the automobile8400, such as a navigation system.

FIG. 30B illustrates an automobile 8500 including the power storagedevice. The automobile 8500 can be charged when the power storage deviceis supplied with electric power through external charging equipment by aplug-in system, a contactless power feeding system, or the like. In FIG.30B, a power storage device included in the automobile 8500 is chargedwith the use of a ground-based charging apparatus 8021 through a cable8022. In charging, a given method such as CHAdeMO (registered trademark)or Combined Charging System may be employed as a charging method, thestandard of a connector, or the like as appropriate. The ground-basedcharging apparatus 8021 may be a charging station provided in a commercefacility or a power source in a house. For example, with the use of aplug-in technique, the power storage device included in the automobile8500 can be charged by being supplied with electric power from outside.The charging can be performed by converting AC electric power into DCelectric power through a converter such as an ACDC converter.

Furthermore, although not illustrated, the vehicle may include a powerreceiving device so that it can be charged by being supplied withelectric power from an above-ground power transmitting device in acontactless manner. In the case of the contactless power feeding system,by fitting a power transmitting device in a road or an exterior wall,charging can be performed not only when the electric vehicle is stoppedbut also when driven. In addition, the contactless power feeding systemmay be utilized to perform transmission and reception of electric powerbetween vehicles. Furthermore, a solar cell may be provided in theexterior of the automobile to charge the power storage device when theautomobile stops or moves. To supply electric power in such acontactless manner, an electromagnetic induction method or a magneticresonance method can be used.

According to one embodiment of the present invention, the power storagedevice can have improved cycle characteristics and reliability.Furthermore, according to one embodiment of the present invention, thepower storage device itself can be made more compact and lightweight asa result of improved characteristics of the power storage device. Thecompact and lightweight power storage device contributes to a reductionin the weight of a vehicle, and thus increases the driving distance.Furthermore, the power storage device included in the vehicle can beused as a power source for supplying electric power to products otherthan the vehicle. In such a case, the use of a commercial power sourcecan be avoided at peak time of electric power demand.

This embodiment can be combined with any of the other embodiments asappropriate.

Example 1

In this example, a power storage device of one embodiment of the presentinvention was fabricated according to Embodiment 1, and subjected to acycle life test at 25° C. together with power storage devices forcomparison.

<Fabrication Methods for Samples>

In this example, the power storage device 500 illustrated in FIG. 1A wasfabricated. Fabrication methods for samples will be described below.

The one embodiment of the present invention was applied to 14 samples,A1, A2, B1, B2, C1, C2, D1, D2, E1, E2, F1, F2, B3, and B4. Forcomparison, 2 comparison samples, a1 and a2 were fabricated.

Lithium bis(pentafluoroethanesulfonyl)amide (LiBETA) and lithiumhexafluorophosphate were used as solutes for the samples A1, A2, B1, B2,C1, C2, D1, D2, E1, E2, F1, F2, B3, and B4. The concentration of lithiumhexafluorophosphate in the samples A1, A2, B1, B2, C1, C2, D1, D2, E1,E2, F1, F2, B3, and B4 differs from one another. Lithiumbis(pentafluoroethanesulfonyl)amide (LiBETA) was used as a solute forthe comparison samples a1 and a2, and lithium hexafluorophosphate wasnot used.

Cellulosic fiber was used for a separator in the samples A1, A2, B1, B2,C1, C2, D1, D2, E1, E2, F1, and F2 and the comparison samples a1 and a2.Polyphenylene sulfide (PPS) was used for the samples B3 and B4.

After the power storage device 500 was manufactured, the samples A2, B2,C2, D2, E2, F2, and B4 and the comparison sample a2 were subjected toheat treatment at 170° C. for 15 minutes. The heat treatment isperformed assuming that the samples and fluorine rubber are integratedas in Embodiment 2. Note that the samples A1, B1, C1, D1, E1, F1, and B3and the comparison sample a1 were not subjected to heat treatment.

Table 1 shows the electrolyte, the separator, and conditions of heattreatment for each sample.

TABLE 1 Electrolyte LiPF₆ Sample Condition [wt %] Separator Heattreatment Comparison Electrolyte a 0.0 Cellulosic fiber — sample a1Comparison 170° C. 15 min sample a2 Sample A1 Electrolyte A 0.18Cellulosic fiber — Sample A2 170° C. 15 min Sample B1 Electrolyte B 0.27Cellulosic fiber — Sample B2 170° C. 15 min Sample C1 Electrolyte C 0.51Cellulosic fiber — Sample C2 170° C. 15 min Sample D1 Electrolyte D 1.1Cellulosic fiber — Sample D2 170° C. 15 min Sample E1 Electrolyte E 2.0Cellulosic fiber — Sample E2 170° C. 15 min Sample F1 Electrolyte F 3.0Cellulosic fiber — Sample F2 170° C. 15 min Sample B3 Electrolyte B 0.27Polyphenylene — Sample B4 sulfide 170° C. 15 min

Manufacturing methods of the electrolyte are described.

The electrolyte a used for the comparison samples a1 and a2 isdescribed. The electrolyte a was formed as follows: vinylene carbonate(VC) was mixed with a mixed solution in which ethylene carbonate (EC)and propylene carbonate (PC) were mixed at a volume ratio of 1:1, andlithium bis(pentafluoroethanesulfonyl)amide (LiBETA) was dissolved inthe solution. The amount of vinylene carbonate (VC) dissolved in theelectrolyte a was 1 wt % in the weight ratio. The amount of lithiumbis(pentafluoroethanesulfonyl)amide (LiBETA) dissolved in theelectrolyte a was 1 mol/L in the molecular concentration.

The electrolyte A used for the samples A1 and A2 is described. Theelectrolyte A was formed as follows: vinylene carbonate (VC) was mixedwith a mixed solution in which ethylene carbonate (EC) and propylenecarbonate (PC) were mixed at a volume ratio of 1:1, and lithiumbis(pentafluoroethanesulfonyl)amide (LiBETA) and lithiumhexafluorophosphate were dissolved in the solution. The amount ofvinylene carbonate (VC) dissolved in the electrolyte A was 1 wt % in theweight ratio. The amount of lithium bis(pentafluoroethanesulfonyl)amide(LiBETA) dissolved in the electrolyte A was 1 mol/L in the molecularconcentration. The amount of lithium hexafluorophosphate dissolved inthe electrolyte A was 0.18 wt % in the weight ratio.

The electrolyte B used for the samples B1, B2, B3, and B4 is described.The electrolyte B was formed as follows: vinylene carbonate (VC) wasmixed with a mixed solution in which ethylene carbonate (EC) andpropylene carbonate (PC) were mixed at a volume ratio of 1:1, andlithium bis(pentafluoroethanesulfonyl)amide (LiBETA) and lithiumhexafluorophosphate were dissolved in the solution. The amount ofvinylene carbonate (VC) dissolved in the electrolyte B was 1 wt % in theweight ratio. The amount of lithium bis(pentafluoroethanesulfonyl)amide(LiBETA) dissolved in the electrolyte B was 1 mol/L in the molecularconcentration. The amount of lithium hexafluorophosphate dissolved inthe electrolyte B was 0.27 wt % in the weight ratio.

The electrolyte C used for the samples C1 and C2 is described. Theelectrolyte C was formed as follows: vinylene carbonate (VC) was mixedwith a mixed solution in which ethylene carbonate (EC) and propylenecarbonate (PC) were mixed at a volume ratio of 1:1, and lithiumbis(pentafluoroethanesulfonyl)amide (LiBETA) and lithiumhexafluorophosphate were dissolved in the solution. The amount ofvinylene carbonate (VC) dissolved in the electrolyte C was 1 wt % in theweight ratio. The amount of lithium bis(pentafluoroethanesulfonyl)amide(LiBETA) dissolved in the electrolyte C was 1 mol/L in the molecularconcentration. The amount of lithium hexafluorophosphate dissolved inthe electrolyte C was 0.51 wt % in the weight ratio.

The electrolyte D used for the samples D1 and D2 is described. Theelectrolyte D was formed as follows: vinylene carbonate (VC) was mixedwith a mixed solution in which ethylene carbonate (EC) and propylenecarbonate (PC) were mixed at a volume ratio of 1:1, and lithiumbis(pentafluoroethanesulfonyl)amide (LiBETA) and lithiumhexafluorophosphate were dissolved in the solution. The amount ofvinylene carbonate (VC) dissolved in the electrolyte D was 1 wt % in theweight ratio. The amount of lithium bis(pentafluoroethanesulfonyl)amide(LiBETA) dissolved in the electrolyte D was 1 mol/L in the molecularconcentration. The amount of lithium hexafluorophosphate dissolved inthe electrolyte D was 1.1 wt % in the weight ratio.

The electrolyte E used for the samples E1 and E2 is described. Theelectrolyte E was formed as follows: vinylene carbonate (VC) was mixedwith a mixed solution in which ethylene carbonate (EC) and propylenecarbonate (PC) were mixed at a volume ratio of 1:1, and lithiumbis(pentafluoroethanesulfonyl)amide (LiBETA) and lithiumhexafluorophosphate were dissolved in the solution. The amount ofvinylene carbonate (VC) dissolved in the electrolyte E was 1 wt % in theweight ratio. The amount of lithium bis(pentafluoroethanesulfonyl)amide(LiBETA) dissolved in the electrolyte E was 1 mol/L in the molecularconcentration. The amount of lithium hexafluorophosphate dissolved inthe electrolyte A was 2.0 wt % in the weight ratio.

The electrolyte F used for the samples F1 and F2 is described. Theelectrolyte F was formed as follows: vinylene carbonate (VC) was mixedwith a mixed solution in which ethylene carbonate (EC) and propylenecarbonate (PC) were mixed at a volume ratio of 1:1, and lithiumbis(pentafluoroethanesulfonyl)amide (LiBETA) and lithiumhexafluorophosphate were dissolved in the solution. The amount ofvinylene carbonate (VC) dissolved in the electrolyte F was 1 wt % in theweight ratio. The amount of lithium bis(pentafluoroethanesulfonyl)amide(LiBETA) dissolved in the electrolyte F was 1 mol/L in the molecularconcentration. The amount of lithium hexafluorophosphate dissolved inthe electrolyte F was 3.0 wt % in the weight ratio.

Lithium battery grade produced by Kishida Chemical Co., Ltd. (productcode number LBG-00798) was used for the mixed solution in which ethylenecarbonate (EC) and propylene carbonate (PC) were mixed at a volume ratioof 1:1. Lithium battery grade produced by Kishida Chemical Co., Ltd.(product code number LBG-84923) was used for vinylene carbonate (VC). Aproduct by IoLiTec Ionic Liquids Technologies Inc. (product numberKI-0016-HP) was used for lithium bis(pentafluoroethanesulfonyl)amide(LiBETA). Lithium battery grade produced by Kishida Chemical Co., Ltd.(product code number LBG-45860) was used for lithium hexafluorophosphate(LiPF₆).

Next, a method for forming a negative electrode is described. The methodfor forming a negative electrode is common to all the samples A1, A2,B1, B2, C1, C2, D1, D2, E1, E2, F1, F2, B3, and B4 and the comparisonsamples a1 and a2.

Spherical natural graphite having a specific surface area of 6.3 m²/gand an average particle size of 15 μm (CGB-15 manufactured by NipponGraphite Industries, Co., Ltd.) was used as a negative electrode activematerial. For a binder, sodium carboxymethyl cellulose (CMC-Na) and SBRwere used. The polymerization degree of CMC-Na that was used was higherthan or equal to 600 and lower than or equal to 800, and the viscosityof a 1 wt % CMC-Na aqueous solution was in the range from 300 mPa·s to500 mPa·s. The compounding ratio of graphite:CMC-Na:SBR was set to97:1:1.5 (wt %).

First, CMC-Na powder and an active material were mixed and then kneadedwith a mixer, so that a first mixture was obtained.

Subsequently, a small amount of water was added to the first mixture andkneading was performed, so that a second mixture was obtained. Here,“kneading” means “mixing something with a high viscosity”.

Then, water was further added and the mixture was kneaded with a mixer,so that a third mixture was obtained.

Then, a 50 wt % SBR aqueous dispersion liquid was added, and mixing wasperformed with a mixer. After that, the obtained mixture was degassedunder a reduced pressure, so that a slurry was obtained.

Subsequently, the slurry was applied to a negative electrode currentcollector with the use of a continuous coater. An 18-μm-thick rolledcopper foil was used as the negative electrode current collector. Thecoating speed was set to 0.75 m/min.

Then, the solvent in the slurry applied to the negative electrodecurrent collector was vaporized in a drying furnace. Vaporizationtreatment was performed at 50° C. in an air atmosphere for 120 secondsand then further performed at 80° C. in the air atmosphere for 120seconds. After that, further vaporization treatment was performed at100° C. under a reduced pressure (−100 kPa) for 10 hours.

Through the above steps, the negative electrode active material layerwas formed on one surface of the negative electrode current collector,so that the negative electrode was fabricated.

Next, a method for forming the positive electrode is described. Themethod for forming a negative electrode is common to all the samples A1,A2, B1, B2, C1, C2, D1, D2, E1, E2, F1, F2, B3, and B4 and thecomparison samples a1 and a2.

LiCoO₂, polyvinylidene fluoride (PVDF), and acetylene black were used asa positive electrode active material, a binder, and a conductiveadditive. LiCoO₂ with a specific surface area of 0.55 m²/g and anaverage particle size of 6.3 μm produced by NIPPON CHEMICAL INDUSTRIALCO., LTD. (C-5hV) was used. The compounding ratio of LiCoO₂:acetyleneblack:PVDF was 95:3:2 (wt %).

Note that the average particle size in this specification and the likeis a cumulative sum value of 50% (D50) on the basis of volume.

First, acetylene black and PVDF were mixed in a mixer, so that a thirdmixture was obtained.

Next, the active material was added to the third mixture, so that afourth mixture was obtained.

After that, a solvent N-methyl-2-pyrrolidone (NMP) was added to thefourth mixture and mixing was performed with a mixer. Through the abovesteps, a slurry was formed.

Then, mixing was performed with a large-sized mixer.

Subsequently, the slurry was applied to a positive electrode currentcollector with the use of a continuous coater. A 20-μm-thick aluminumcurrent collector was used as the positive electrode current collector.The coating speed was set to 0.2 m/min.

Then, the solvent in the slurry applied to the positive electrodecurrent collector was vaporized in a drying furnace. Solventvaporization treatment was performed at 70° C. in an air atmosphere for7.5 minutes and then further performed at 90° C. in the air atmospherefor 7.5 minutes.

After that, heat treatment was performed in a reduced-pressureatmosphere (at a gauge pressure of −100 kPa) at 170° C. for 10 hours.Subsequently, the positive electrode active material layer was pressedby a roll press method so as to be consolidated.

Through the above steps, the positive electrode active material layerwas formed on one surface of the positive electrode current collector,so that the positive electrode was fabricated.

Table 2 lists the averages of the active material loadings, thethicknesses, and the densities of each of the positive electrode activematerial layers, and Table 3 lists those for the negative electrodeactive material layers. The values shown in this specification are theaverages of measurement values of each of the electrodes used infabricating the samples. Note that when the active material layers wereformed on opposite surfaces of the current collector, the values are theaverages of the active material loadings, the thicknesses, and thedensities of the active material layer on one surface of the currentcollector.

The content was calculated from the area and weight of electrodesweighed by an electronic balance. The density was calculated from thethickness measured by a micrometer.

TABLE 2 Positive electrode Loading Thickness Density Sample [mg/cm²][μm] [g/cm³] Comparison 21.1 66 3.20 sample a1 Comparison 21.2 67 3.16sample a2 Sample A1 21.4 66 3.25 Sample A2 21.5 66 3.25 Sample B1 21.370 3.05 Sample B2 21.3 67 3.18 Sample C1 21.3 66 3.22 Sample C2 21.3 663.22 Sample D1 21.3 66 3.23 Sample D2 21.3 66 3.23 Sample E1 21.6 673.23 Sample E2 21.6 68 3.18 Sample F1 21.3 65 3.28 Sample F2 21.4 673.19 Sample B3 24.4 72 3.39 Sample B4 24.4 71 3.44

TABLE 3 Negative electrode Loading Thickness Density Sample [mg/cm²][mm] [g/cm³] Comparison 9.5 107 0.89 sample a1 Comparison 9.7 101 0.96sample a2 Sample A1 10.2 106 0.96 Sample A2 10.2 103 0.99 Sample B1 9.799 0.98 Sample B2 9.8 105 0.93 Sample C1 10.1 103 0.98 Sample C2 10.1102 0.99 Sample D1 10.1 107 0.95 Sample D2 10.2 102 1.00 Sample E1 10.3109 0.95 Sample E2 10.4 103 1.01 Sample F1 10.2 106 0.96 Sample F2 10.2108 0.94 Sample B3 9.7 101 0.96 Sample B4 9.7 103 0.95

Next, the method for manufacturing the power storage device isdescribed. The samples fabricated in this example each include onepositive electrode in which a positive electrode active material layeris provided on one surface of a positive electrode current collector andone negative electrode in which a negative electrode active materiallayer is provided on one surface of a negative electrode currentcollector. In other words, the samples in this example each include onepositive electrode active material layer and one negative electrodeactive material layer.

First, a positive electrode, a negative electrode, and a first separatorwere cut. The size of the positive electrode, negative electrode, andfirst separator were respectively 20.49 cm², 23.84 cm², and 24.75 cm².

Cellulosic fiber was used for a separator in the samples A1, A2, B1, B2,C1, C2, D1, D2, E1, E2, F1, and F2 and the comparison samples a1 and a2.Specifically, 30-μm-thick solvent-spun regenerated cellulosic fiber(product number TF40) produced by NIPPON KODOSHI CORPORATION was used.Polyphenylene sulfide was used for the samples B₃ and B₄. Specifically,a stack of two 46-μm-thick polyphenylene sulfide paper (product numberPS0020) produced by Toray Industries, Inc. was used.

Then, the positive electrode active material and the negative electrodeactive material in tab regions were removed to expose the currentcollectors.

After that, the positive electrode and the negative electrode werestacked with the first separator therebetween. At this time, thepositive electrode and the negative electrode were stacked such that thepositive electrode active material layer and the negative electrodeactive material layer faced each other.

Then, leads were bonded to the positive electrode and the negativeelectrode by ultrasonic welding.

Next, the region where the electrodes are stacked and the lead is bondedwas wrapped with a second separator. This can prevent dissolution of theresin layer of the exterior body by heat treatment performed later andcontact of aluminum of the exterior body with the electrodes. The sizeof the second separator was 104 cm².

Then, facing parts of two of four sides of the exterior body were bondedto each other by heating.

As an exterior body, an aluminum film with opposite surfaces coveredwith a resin layer was used.

After that, sealing layers provided for the leads were positioned so asto overlap with a sealing layer of the exterior body, and bonding wasperformed by heating. At this time, facing parts of a side of theexterior body except a side used for introduction of an electrolytesolution were bonded to each other.

Next, heat treatment for drying the exterior body and the positiveelectrode, the separator, and the negative electrode wrapped by theexterior body was performed in a reduced-pressure atmosphere (at a gaugepressure of −100 kPa) at 80° C. for 10 hours.

Next, in an argon gas atmosphere, an approximately 600 μL of electrolytewas introduced from one side of the exterior body that was not sealed.The electrolyte a was introduced to the comparison samples a1 and a2.The electrolyte A was introduced to the samples A1 and A2. Theelectrolyte B was introduced to the samples B1 to B4. The electrolyte Cwas introduced to the samples C1 and C2. The electrolyte D wasintroduced to the samples D1 and D2. The electrolyte E was introduced tothe samples E1 and E2. The electrolyte F was introduced to the samplesF1 and F2.

After that, the one side of the exterior body was sealed by heating in areduced-pressure atmosphere (at a gauge pressure of −100 kPa). Throughthe above steps, each thin storage battery was fabricated.

Next, the samples A2, B2, C2, D2, E2, F2, and B4 and the comparisonsample a2 were subjected to heat treatment. Assuming that each sampleand fluorine rubber are integrally formed as in Embodiment 2, heattreatment was performed in an atmospheric pressure atmosphere at 170° C.for 15 minutes. Specifically, the temperature of a thermostatic bath wasraised to approximately 170° C., each sample was put in the thermostaticbath, and after 15 minutes, the sample was taken out. Expansionaccompanying the heat treatment did not occur in the exterior body ofeach sample.

Through the above, the samples A1, A2, B1, B2, C1, C2, D1, D2, E1, E2,F1, F2, B3, and B4 and the comparison samples a1 and a2 were fabricated.

<Measurement of Charge and Discharge Characteristics>

Next, the charge and discharge characteristics at 25° C. of the samplesin this example were measured. The measurement was performed with acharge-discharge measuring instrument (produced by TOYO SYSTEM Co.,LTD.). Constant current-constant voltage charging was performed untilthe voltage reached an upper voltage limit of 4.3 V, and constantcurrent discharging was performed until the voltage reached a lowervoltage limit of 2.5 V. The charging and discharging were performed 3cycles at a rate of 0.1 C, and then, a long-term cycle test wasperformed at a rate of 0.3 C. A 10-minute break was taken after thecharging and discharging.

Note that the rates were calculated using 170 mAh/g, which is capacityobtained when the upper charging voltage limit of LiCoO₂ serving as thepositive electrode active material, is 4.3 V, as a reference.

FIG. 31A shows charge-discharge curves of the comparison sample a1, FIG.31B for the comparison sample a2, FIG. 31C for the sample A1, FIG. 31Dfor the sample A2, FIG. 32A for the sample B1, FIG. 32B for the sampleB2, FIG. 32C for the sample C1, FIG. 32D for the sample C2, FIG. 33A forthe sample D1, FIG. 33B for the sample D2, FIG. 33C for the sample E1,FIG. 33D for the sample E2, FIG. 34A for the sample F1, FIG. 34B for thesample F2, FIG. 34C for the sample B3, and FIG. 34D for the sample B4.In FIGS. 31A to 34D, the horizontal axis and the vertical axis representcapacity [mAh/g] and voltage [V], respectively. Charge and dischargecharacteristics at the 1st cycle and the 300th cycle are shown for eachsample. The capacity is per weight of the positive electrode activematerial.

It was found that in the samples subjected to heat treatment at 170° C.for 15 minutes, repetitive charge and discharge resulted in a decreasein capacity. It was shown that a reduction in capacity after repetitivecharge and discharge of the samples A2, B2, C2, D2, and B4 containinglithium hexafluorophosphate, which were embodiments of the presentinvention, was suppressed as compared to the comparison sample a2containing no lithium hexafluorophosphate.

FIG. 35A shows cycle characteristics of the discharge capacity of thecomparison samples a1 and a2, FIG. 35B for the samples A1 and A2, FIG.35C for the samples B1 and B2, FIG. 35D for the samples C1 and C2, FIG.36A for the samples D1 and D2, FIG. 36B for the samples E1 and E2, FIG.36C for the samples F1 and F2, and FIG. 36D for the samples B3 and B4.In FIGS. 35A to 35D and FIGS. 36A to 36D, the horizontal axis and thevertical axis represent cycles [times] and capacity [mAh/g],respectively.

FIG. 37A shows cycle characteristics of discharge capacity retentionrate of the comparison samples a1 and a2, FIG. 37B for the samples A1and A2, FIG. 37C for the samples B1 and B2, FIG. 37D for the samples C1and C2, FIG. 38A for the samples D1 and D2, FIG. 38B for the samples E1and E2, FIG. 38C for the samples F1 and F2, and FIG. 38D for the samplesB3 and B4. In FIGS. 37A to 37D and FIGS. 38A to 38D, the horizontal axisand the vertical axis represent cycles [times] and capacity retentionrate [%], respectively. The capacity retention rate is percentage ofdischarge capacity for each cycle time with respect to the maximum valueof the discharge capacity of each sample.

It was found that in the samples subjected to heat treatment at 170° C.for 15 minutes, repetitive charge and discharge resulted in a decreasein capacity. It was shown that a reduction in capacity after repetitivecharge and discharge of the samples A2, B2, C2, D2, and B4 containinglithium hexafluorophosphate, which were embodiments of the presentinvention, was suppressed as compared to the comparison sample a2containing no lithium hexafluorophosphate.

In this embodiment, it was found that lithiumbis(pentafluoroethanesulfonyl)amide (LiBETA) with high thermalresistance mainly supplies lithium ions which serve as carrier ions,which can prevent a reduction in capacity after repetitive charge anddischarge even when heat treatment was performed. In addition, owing tolithium hexafluorophosphate, a passivation film was formed on thesurface aluminum used for a positive electrode current collector in heattreatment at 170° C. or charging and discharging, so that favorablecycle characteristics were kept. Note that the capacity of the samplesE2 and F2 was much reduced as compared to the comparison sample a2.Since the concentration of lithium hexafluorophosphate in each of thesamples E2 and F2 was high, lithium hexafluorophosphate was decomposedinto LiF and PF₅ by the heat treatment at 170° C., and PF₅ decomposedthe solvent, which probably caused degradation of batterycharacteristics such as a reduction in capacity. In other words, it wasfound that lithium hexafluorophosphate (LiPF₆) was desirably used at anamount enough to form a passivation film on the current collectorsurface.

In one embodiment of the present invention, since lithiumbis(pentafluoroethanesulfonyl)amide (LiBETA) and lithiumhexafluorophosphate (LiPF₆) were used as solutes, the capacity is lesslikely to be reduced after repetitive charging and discharging even whenthe power storage battery was subjected to heat treatment.

FIG. 39A shows cycle characteristics of the energy density of thecomparison samples a1 and a2, FIG. 39B for the samples A1 and A2, FIG.39C for the samples B1 and B2, FIG. 39D for the samples C1 and C2, FIG.40A for the samples D1 and D2, FIG. 40B for the samples E1 and E2, FIG.40C for the samples F1 and F2, and FIG. 40D for the samples B3 and B4.In FIGS. 39A to 39D and FIGS. 40A to 40D, the horizontal axis and thevertical axis represent cycles [times] and energy density [mWh/g],respectively. The energy density is the product of discharge capacityand voltage shown in the discharge curves in FIGS. 31A to 31D, FIGS. 32Ato 32D, FIGS. 33A to 33D, and FIGS. 34A to 34D.

FIG. 41A shows cycle characteristics of the energy density of thecomparison samples a1 and a2, FIG. 41B for the samples A1 and A2, FIG.41C for the samples B1 and B2, FIG. 41D for the samples C1 and C2, FIG.42A for the samples D1 and D2, FIG. 42B for the samples E1 and E2, FIG.42C for the samples F1 and F2, and FIG. 42D for the samples B3 and B4.In FIGS. 41A to 41D and FIGS. 42A to 42D, the horizontal axis and thevertical axis represent cycles [times] and energy density retention rate[%], respectively. The energy density retention rate is percentage ofenergy density for each cycle time with respect to the maximum value ofthe energy density of each sample.

It was found that in the samples subjected to heat treatment at 170° C.for 15 minutes, repetitive charge and discharge resulted in a decreasein energy density. It was shown that a reduction in energy density afterrepetitive charge and discharge of the samples A2, B2, C2, D2, and B4containing lithium hexafluorophosphate, which were embodiments of thepresent invention, was suppressed as compared to the comparison samplea2 containing no lithium hexafluorophosphate.

In this embodiment, it was found that lithiumbis(pentafluoroethanesulfonyl)amide (LiBETA) with high thermalresistance mainly supplies lithium ions which serve as carrier ions,which can prevent a reduction in energy density after repetitive chargeand discharge even when heat treatment was performed. In addition, owingto lithium hexafluorophosphate, a passivation film was formed on thesurface aluminum used for a positive electrode current collector in heattreatment at 170° C. or charging and discharging, so that favorablecycle characteristics were kept. Note that the energy density of thesamples E2 and F2 was much reduced as compared to the comparison samplea2. Since the concentration of lithium hexafluorophosphate in each ofthe samples E2 and F2 was high, lithium hexafluorophosphate wasdecomposed into LiF and PF₅ by the heat treatment at 170° C., and PF₅decomposed the solvent, which probably caused degradation of batterycharacteristics such as a reduction in energy density. In other words,it was found that lithium hexafluorophosphate (LiPF₆) was desirably usedat an amount enough to form a passivation film on the current collectorsurface.

In one embodiment of the present invention, since lithiumbis(pentafluoroethanesulfonyl)amide (LiBETA) and lithiumhexafluorophosphate (LiPF₆) were used as solutes, the energy density isless likely to be reduced after repetitive charging and discharging evenwhen the power storage battery was subjected to heat treatment.

Table 3 and FIG. 43A show the relationship between the concentration ofLiPF₆ with respect to the electrolyte and the capacity retention ratefor each of the comparison sample a1 and the samples A2, B2, C2, D2, E2,F2, and B4 which were subjected to heat treatment at 170° C. In FIG.43A, the horizontal axis and the vertical axis represent the LiPf₆concentration [wt %] and capacity retention [%], respectively. In FIG.43A, black circles indicate data of the comparison sample a2 and thesamples A2, B2, C2, D2, E2, and F2 in which cellulosic fiber was usedfor the separator, and crosses indicate data of the sample B4 in whichpolyphenylene sulfide was used as the separator. The capacity retentionrate is a value at the 300th cycle for each sample shown in FIGS. 37A to37D and FIGS. 38A to 38D.

Table 4 and FIG. 43B show the relationship between the concentration ofLiPF₆ with respect to the electrolyte and the energy density for each ofthe comparison sample a2 and the samples A2, B2, C2, D2, E2, F2, and B4which were subjected to heat treatment at 170° C. for 15 minutes. InFIG. 43B, the horizontal axis and the vertical axis represent the LiPf₆concentration [wt %] and energy density [%], respectively. In FIG. 43B,black circles indicate data of the comparison sample a2 and the samplesA2, B2, C2, D2, E2, and F2 in which cellulosic fiber was used for theseparator, and crosses indicate data of the sample B4 in whichpolyphenylene sulfide was used as the separator. The energy densityretention rate is a value at the 300th cycle for each sample shown inFIGS. 41A to 41D and FIGS. 42A to 42D.

TABLE 4 Electrolyte Capacity Energy LiPF₆ retention retention SampleCondition [wt %] Separator rate [%] rate [%] Comparison Electrolyte a0.0 Cellulosic fiber 89.8 87.9 sample a1 Comparison 84.3 81.8 sample a2Sample A1 Electrolyte A 0.18 Cellulosic fiber 87.9 86.6 Sample A2 89.088.3 Sample B1 Electrolyte B 0.27 Cellulosic fiber 88.7 87.3 Sample B288.9 88.0 Sample C1 Electrolyte C 0.51 Cellulosic fiber 89.3 88.6 SampleC2 85.8 84.1 Sample D1 Electrolyte D 1.1 Cellulosic fiber 89.4 88.8Sample D2 84.9 82.3 Sample E1 Electrolyte E 2.0 Cellulosic fiber 89.689.0 Sample E2 79.0 73.8 Sample F1 Electrolyte F 3.0 Cellulosic fiber88.5 87.9 Sample F2 71.4 59.9 Sample B3 Electrolyte B 0.27 Polyphenylene85.0 82.7 Sample B4 sulfide 86.0 84.7

As shown in Table 4 and FIGS. 43A and 43B, the capacity retention rateand energy retention rate after repetitive charge and discharge of thesamples A2, B2, C2, D2, and B4 which were embodiments of the presentinvention were higher than those of the comparison sample a2 containingno lithium hexafluorophosphate. In contrast, the capacity retention rateand energy retention rate of the samples E2 and F2 having high LiPF₆concentrations were decreased.

These results show that the electrolyte preferably contains ethylenecarbonate (EC), propylene carbonate (PC), vinylene carbonate (VC),lithium bis(pentafluoroethanesulfonyl)amide (LiBETA), and lithiumhexafluorophosphate. It was also shown that the amount of lithiumhexafluorophosphate dissolved in the electrolyte is preferably more thanor equal to 0.01 wt % and less than or equal to 1.9 wt %, furtherpreferably more than or equal to 0.05 wt % and less than or equal to 1.2wt %, still further more than or equal to 0.1 wt % and less than orequal to 0.8 wt %.

It was shown that the use of lithium bis(pentafluoroethanesulfonyl)amide(LiBETA) and lithium hexafluorophosphate as electrolyte solutes canproduce a power storage battery with high thermal resistance.

Example 2

In this example, a power storage device of one embodiment of the presentinvention was fabricated on the basis of Embodiment 1, and subjected toa cycle life test at 45° C. together with power storage devices forcomparison.

<Fabrication Methods for Samples>

In this example, the power storage device 500 illustrated in FIG. 1A wasfabricated. Fabrication methods for samples will be described below.

The one embodiment of the present invention was applied to 4 samples,G1, G2, H1, and H2. For comparison, 2 comparison samples, a3 and a4 werefabricated.

Lithium bis(pentafluoroethanesulfonyl)amide (LiBETA) and lithiumhexafluorophosphate were used as solutes for the samples G1, G2, H1, andH2. The concentration of lithium hexafluorophosphate in the samples G1,G2, H1, and H2 differs from one another. Lithiumbis(pentafluoroethanesulfonyl)amide (LiBETA) was used as a solute forthe comparison samples a3 and a4, and lithium hexafluorophosphate wasnot used.

Cellulosic fiber was used for a separator in the samples G1, G2, H1, andH2 and the comparison samples a3 and a4.

After the power storage device 500 was manufactured, the samples G2 andH2 and the comparison sample a4 were subjected to heat treatment at 170°C. for 15 minutes. The heat treatment was performed assuming that thesamples and fluorine rubber are integrated as in Embodiment 2. Note thatthe samples G1 and H1 and the comparison sample a3 were not subjected toheat treatment.

Table 5 shows the electrolyte, the separator, and conditions of heattreatment for each sample.

TABLE 5 Electrolyte LiPF₆ Sample Condition [wt %] Separator Heattreatment Comparison Electrolyte a 0.0 Cellulosic fiber — sample a3Comparison 170° C. 15 min sample a4 Sample G1 Electrolyte G 0.26Cellulosic fiber — Sample G2 170° C. 15 min Sample H1 Electrolyte H 0.51Cellulosic fiber — Sample H2 170° C. 15 min

Manufacturing methods of the electrolyte are described.

The electrolyte a was used as an electrolyte of the comparison samplesa3 and a4. The description of the electrolyte a is omitted here becauseExample 1 can be referred to.

The electrolyte G used for the samples G1 and G2 is described. Theelectrolyte G was formed as follows: vinylene carbonate (VC) was mixedwith a mixed solution in which ethylene carbonate (EC) and propylenecarbonate (PC) were mixed at a volume ratio of 1:1, and lithiumbis(pentafluoroethanesulfonyl)amide (LiBETA) and lithiumhexafluorophosphate were dissolved in the solution. The amount ofvinylene carbonate (VC) dissolved in the electrolyte G was 1 wt % in theweight ratio. The amount of lithium bis(pentafluoroethanesulfonyl)amide(LiBETA) dissolved in the electrolyte G was 1 mol/L in the molecularconcentration. The amount of lithium hexafluorophosphate dissolved inthe electrolyte G was 0.26 wt % in the weight ratio.

The electrolyte H used for the samples H1 and H2 is described. Theelectrolyte H was formed as follows: vinylene carbonate (VC) was mixedwith a mixed solution in which ethylene carbonate (EC) and propylenecarbonate (PC) were mixed at a volume ratio of 1:1, and lithiumbis(pentafluoroethanesulfonyl)amide (LiBETA) and lithiumhexafluorophosphate were dissolved in the solution. The amount ofvinylene carbonate (VC) dissolved in the electrolyte H was 1 wt % in theweight ratio. The amount of lithium bis(pentafluoroethanesulfonyl)amide(LiBETA) dissolved in the electrolyte H was 1 mol/L in the molecularconcentration. The amount of lithium hexafluorophosphate dissolved inthe electrolyte H was 0.51 wt % in the weight ratio.

Lithium battery grade produced by Kishida Chemical Co., Ltd. (productcode number LBG-00798) was used for the mixed solution in which ethylenecarbonate (EC) and propylene carbonate (PC) were mixed at a volume ratioof 1:1. Lithium battery grade produced by Kishida Chemical Co., Ltd.(product code number LBG-84923) was used for vinylene carbonate (VC). Aproduct by IoLiTec Ionic Liquids Technologies Inc. (product numberKI-0016-HP) was used for lithium bis(pentafluoroethanesulfonyl)amide(LiBETA). Lithium battery grade produced by Kishida Chemical Co., Ltd.(product code number LBG-45860) was used for lithiumhexafluorophosphate.

Next, a negative electrode was formed. Since Example 1 can be referredto for the formation method of the negative electrode, the descriptionis omitted here.

Next, a positive electrode was formed. Since Example 1 can be referredto for the formation method of the negative electrode, the descriptionis omitted here.

Table 6 lists the averages of the active material loadings, thethicknesses, and the densities of each of the positive electrode activematerial layers, and Table 7 lists those for the negative electrodeactive material layers.

TABLE 6 Positive electrode Loading Thickness Density Sample [mg/cm²][μm] [g/cm³] Comparison 21.2 67 3.16 sample a3 Comparison 21.2 66 3.21sample a4 Sample G1 21.3 66 3.23 Sample G2 21.3 66 3.23 Sample H1 21.567 3.20 Sample H2 21.5 66 3.26

TABLE 7 Negative electrode Loading Thickness Density Sample [mg/cm²][mm] [g/cm³] Comparison 9.7 106 0.91 sample a3 Comparison 9.7 99 0.98sample a4 Sample G1 9.9 101 0.98 Sample G2 9.9 105 0.95 Sample H1 10.3106 0.97 Sample H2 10.3 110 0.93

Next, the method for manufacturing the power storage device isdescribed. The samples fabricated in this example each include onepositive electrode in which a positive electrode active material layeris provided on one surface of a positive electrode current collector andone negative electrode in which a negative electrode active materiallayer is provided on one surface of a negative electrode currentcollector. In other words, the samples in this example each include onepositive electrode active material layer and one negative electrodeactive material layer.

First, a positive electrode, a negative electrode, and a first separatorwere cut. The size of the positive electrode, negative electrode, andfirst separator were respectively 20.49 cm², 23.84 cm², and 24.75 cm².

Cellulosic fiber was used for a separator in the samples G1, G2, H1, andH2 and the comparison samples a3 and a4. Specifically, 30-μm-thicksolvent-spun regenerated cellulosic fiber (product number TF40) producedby NIPPON KODOSHI CORPORATION was used.

Then, the positive electrode active material and the negative electrodeactive material in tab regions were removed to expose the currentcollectors.

After that, the positive electrode and the negative electrode werestacked with the first separator therebetween. At this time, thepositive electrode and the negative electrode were stacked such that thepositive electrode active material layer and the negative electrodeactive material layer faced each other.

Then, leads were bonded to the positive electrode and the negativeelectrode by ultrasonic welding.

Next, the region where the electrodes are stacked and the lead is bondedwas wrapped with a second separator. This can prevent dissolution of theresin layer of the exterior body by heat treatment performed later andcontact of aluminum of the exterior body with the electrodes. The sizeof the second separator was 104 cm².

Then, facing parts of two of four sides of the exterior body were bondedto each other by heating.

As an exterior body, an aluminum film with opposite surfaces coveredwith a resin layer was used.

After that, sealing layers provided for the leads were positioned so asto overlap with a sealing layer of the exterior body, and bonding wasperformed by heating. At this time, facing parts of a side of theexterior body except a side used for introduction of an electrolytesolution were bonded to each other.

Next, heat treatment for drying the exterior body and the positiveelectrode, the separator, and the negative electrode wrapped by theexterior body was performed in a reduced-pressure atmosphere (at a gaugepressure of −100 kPa) at 80° C. for 10 hours.

Next, in an argon gas atmosphere, an approximately 600 μL of electrolytewas introduced from one side of the exterior body that was not sealed.The electrolyte a was introduced to the comparison samples a3 and a4.The electrolyte G was introduced to the samples G1 and G2. Theelectrolyte H was introduced to the samples H1 and H2.

After that, the one side of the exterior body was sealed by heating in areduced-pressure atmosphere (at a gauge pressure of −100 kPa). Throughthe above steps, each thin storage battery was fabricated.

Next, the samples G2 and H2 and the comparison sample a4 were subjectedto heat treatment. Assuming that each sample and fluorine rubber areintegrally formed as in Embodiment 2, heat treatment was performed in anatmospheric pressure atmosphere at 170° C. for 15 minutes. Specifically,the temperature of a thermostatic bath was raised to approximately 170°C., each sample was put in the thermostatic bath, and after 15 minutes,the sample was taken out. Expansion accompanying the heat treatment didnot occur in the exterior body of each sample.

Through the above, the samples G1, G2, H1, and H2 and the comparisonsamples a3 and a4 were fabricated.

<Measurement of Charge and Discharge Characteristics>

Next, the charge and discharge characteristics at 45° C. of the samplesin this example were measured. The measurement was performed with acharge-discharge measuring instrument (produced by TOYO SYSTEM Co.,LTD.). Constant current-constant voltage charging was performed untilthe voltage reached an upper voltage limit of 4.3 V, and constantvoltage discharging was performed until the voltage reached a lowervoltage limit of 2.5 V. The charging and discharging were performed at arate of 0.1 C, and a 10-minute break was taken after the charging. Thecharging and discharging were performed 2 cycles.

Note that the rates were calculated using 170 mAh/g, which is capacityobtained when the upper charging voltage limit of LiCoO₂ serving as thepositive electrode active material, is 4.3 V, as a reference.

FIG. 44A shows charge-discharge curves of the comparison sample a3, FIG.44B for the comparison sample a4, FIG. 44C for the sample G1, FIG. 44Dfor the sample G2, FIG. 45A for the sample H1, and FIG. 45B for thesample H2. In FIGS. 44A to 44D and FIGS. 45A and 45B, the horizontalaxis and the vertical axis represent capacity [mAh/g] and voltage [V],respectively. Charge and discharge characteristics at the 1st cycle andthe 300th cycle are shown for each sample. The capacity is per weight ofthe positive electrode active material.

Note that the 300th-cycle capacity of the comparison samples a3 and a4containing no lithium hexafluorophosphate was close to zero.

FIG. 46A shows cycle characteristics of discharge capacity of thecomparison samples a3 and a4, FIG. 46B for the samples G1 and G2, andFIG. 46C for the samples H1 and H2. In FIGS. 46A to 46C, the horizontalaxis and the vertical axis represent cycles [times] and capacity[mAh/g], respectively.

FIG. 47A shows cycle characteristics of discharge capacity retentionrate of the comparison samples a3 and a4, FIG. 47B for the samples G1and G2, and FIG. 47C for the samples H1 and H2. In FIGS. 47A to 47C, thehorizontal axis and the vertical axis represent cycles [times] andcapacity retention rate [%], respectively. The capacity retention rateis percentage of discharge capacity for each cycle time with respect tothe maximum value of the discharge capacity of each sample.

About-150th-cycle capacity of the comparison samples a3 and a4containing no lithium hexafluorophosphate was close to zero. It seemsthat repetitive charge and discharge at 45° C. causes corrosion ofaluminum used as the positive electrode current collector by lithiumbis(pentafluoroethanesulfonyl)amide (LiBETA). Capacity decrease of thesamples G1, G2, H1, and H2, which were embodiments of the presentinvention, was suppressed even after repetitive charge and dischargecycle, as compared to the comparison samples a3 and a4. In particular,although the sample H2 was subjected to the heat treatment at 170° C.for 15 minutes, it showed high capacity retention rate. This is probablybecause a passivation film was formed on the aluminum surface by lithiumhexafluorophosphate, resulting in suppressing corrosion of aluminum evenwhen the heat treatment was performed.

FIG. 48A shows cycle characteristics of energy density of the comparisonsamples a3 and a4, FIG. 48B for the samples G1 and G2, and FIG. 48C forthe samples H1 and H2. In FIGS. 48A to 48C, the horizontal axis and thevertical axis represent cycles [times] and energy density [mWh/g],respectively. The energy density is the product of discharge capacityand voltage shown in the discharge curves in FIGS. 44A to 44D and FIGS.45A and 45B.

FIG. 49A shows cycle characteristics of energy density retention rate ofthe comparison samples a3 and a4, FIG. 49B for the samples G1 and G2,and FIG. 49C for the samples H1 and H2. In FIGS. 49A to 49C, thehorizontal axis and the vertical axis represent cycles [times] andenergy density retention rate [%], respectively. The energy densityretention rate is percentage of energy density for each cycle time withrespect to the maximum value of the energy density of each sample.

About-150th-cycle energy density of the comparison samples a3 and a4containing no lithium hexafluorophosphate was close to zero. Capacitydecrease of the samples G1, G2, H1, and H2, which were embodiments ofthe present invention, was suppressed even after repetitive charge anddischarge cycle, as compared to the comparison samples a3 and a4. Inparticular, although the sample H2 was subjected to the heat treatmentat 170° C. for 15 minutes, it showed high energy density retention rate.

Table 5 and FIG. 50A show the relationship between the concentration ofLiPF₆ with respect to the electrolyte and the capacity retention ratefor each of the comparison sample a3 and the samples G2 and H2 whichwere subjected to heat treatment at 170° C. In FIG. 50A, the horizontalaxis and the vertical axis represent the LiPf₆ concentration [wt %] andcapacity retention [%], respectively. The capacity retention rate is avalue at the 300th cycle for each sample shown in FIGS. 47A to 47C.

Table 8 and FIG. 50B show the relationship between the concentration ofLiPF₆ with respect to the electrolyte and the energy density for each ofthe comparison sample a4 and the samples G2 and H4 which were subjectedto heat treatment at 170° C. for 15 minutes. In FIG. 50B, the horizontalaxis and the vertical axis represent the LiPf₆ concentration [wt %] andenergy density [%], respectively. The energy density retention rate is avalue at the 300th cycle for each sample shown in FIGS. 49A to 49C.

TABLE 8 Electrolyte Capacity Energy LiPF₆ retention retention SampleCondition [wt %] Separator rate [%] rate [%] Comparison Electrolyte a0.0 Cellulosic 0.105 0.081 sample a3 fiber Comparison 0.051 0.041 samplea4 Sample G1 Electrolyte G 0.26 Cellulosic 87.0 85.7 Sample G2 fiber76.0 67.3 Sample H1 Electrolyte H 0.51 Cellulosic 88.1 87.5 Sample H2fiber 87.6 85.2

As shown in Table 8 and FIGS. 50A and 50B, the capacity retention rateand energy retention rate after repetitive charge and discharge of thesamples G2 and H2 which were embodiments of the present invention werehigher than those of the comparison sample a4 containing no lithiumhexafluorophosphate.

These results show that the electrolyte preferably contains ethylenecarbonate (EC), propylene carbonate (PC), vinylene carbonate (VC),lithium bis(pentafluoroethanesulfonyl)amide (LiBETA), and lithiumhexafluorophosphate. It was also shown that the amount of lithiumhexafluorophosphate dissolved in the electrolyte is preferably more thanor equal to 0.01 wt % and less than or equal to 1.9 wt %, furtherpreferably more than or equal to 0.05 wt % and less than or equal to 1.2wt %, still further more than or equal to 0.1 wt % and less than orequal to 0.8 wt %.

It was shown that the use of lithium bis(pentafluoroethanesulfonyl)amide(LiBETA) and lithium hexafluorophosphate as electrolyte solutions canproduce a power storage battery with high thermal resistance andfavorable charge and discharge characteristics even in high temperatureenvironment (45° C.).

Example 3

In this example, reaction of an electrolyte and aluminum used as thepositive electrode current collector will be described.

<Methods of Fabricating Samples>

In this example, a power storage battery which was one embodiment of thepresent invention was formed and subjected to heat treatment at 170° C.for 15 minutes. After that, the positive electrode was taken out of thepower storage battery, and the aluminum surface of the positiveelectrode was subjected to composition analysis by XPS measurement. Thepower storage battery subjected to heat treatment at 170° C. for 15minutes is referred to as a power storage battery 1. The positiveelectrode taken out of the power storage battery 1 is referred to as apositive electrode 1. For comparison, a sample (a comparison positiveelectrode 1) in which assembly of a power storage battery and heattreatment were not performed after a positive electrode is formed wassubjected to XPS analysis.

A fabrication method of the positive electrode is described. The samefabrication method was used to fabricate the comparison positiveelectrode 1 and the power storage battery 1 (positive electrode 1).

As each positive electrode active material, LiCoO₂ with a specificsurface area of 0.21 m²/g and an average particle size of 10 μm wasused. As each binder, polyvinylidene fluoride (PVDF) was used. As eachconductive additive, acetylene black was used. The compounding ratio ofLiCoO₂:acetylene black:PVDF was set to 95:3:2 (wt %).

First, acetylene black and PVDF were mixed in a mixer, so that a fifthmixture was obtained.

Next, the active material was added to the fifth mixture, so that asixth mixture was obtained.

After that, a solvent N-methyl-2-pyrrolidone (NMP) was added to thefourth mixture and mixing was performed with a mixer. Through the abovesteps, a slurry was formed.

Then, mixing was performed with a large-sized mixer.

Subsequently, the slurry was applied to a positive electrode currentcollector with the use of a continuous coater. A 20-μm-thick aluminumcurrent collector was used as the positive electrode current collector.The coating speed was set to 0.2 m/min.

Then, the solvent in the slurry applied to the positive electrodecurrent collector was vaporized in a drying furnace. Solventvaporization treatment was performed at 70° C. in an air atmosphere for7.5 minutes and then further performed at 90° C. in the air atmospherefor 7.5 minutes.

After that, heat treatment was performed in a reduced-pressureatmosphere (at a gauge pressure of −100 kPa) at 170° C. for 10 hours.Subsequently, the positive electrode active material layer was pressedby a roll press method so as to be consolidated.

Through the above steps, the positive electrode active material layerwas formed on one surface of the positive electrode current collector.

A sample fabricated in the above manner was used as the comparativepositive electrode 1.

Next, a fabrication method of the power storage battery 1 is described.

A fabrication method of the negative electrode is described.

Spherical natural graphite having a specific surface area of 6.3 m²/gand an average particle size of 15 μm (CGB-15 manufactured by NipponGraphite Industries, Co., Ltd.) was used as a negative electrode activematerial. For a binder, sodium carboxymethyl cellulose (CMC-Na) and SBRwere used. The polymerization degree of CMC-Na that was used was higherthan or equal to 600 and lower than or equal to 800, and the viscosityof a 1 wt % CMC-Na aqueous solution was in the range from 300 mPa·s to500 mPa·s. The compounding ratio of graphite:CMC-Na:SBR was set to97:1.5:1.5 (wt %).

First, CMC-Na powder and an active material were mixed and then kneadedwith a mixer, so that a seventh mixture was obtained.

Subsequently, a small amount of water was added to the seventh mixtureand kneading was performed, so that an eighth mixture was obtained.Here, “kneading” means “mixing something with a high viscosity”.

Then, water was further added and the mixture was kneaded with a mixer,so that an eighth mixture was obtained.

Then, a 50 wt % SBR aqueous dispersion liquid was added to the thirdmixture, and mixing was performed with a mixer. After that, the obtainedmixture was degassed under a reduced pressure, so that a slurry wasobtained.

Subsequently, the slurry was applied to a negative electrode currentcollector with the use of a continuous coater. An 18-μm-thick rolledcopper foil was used as the negative electrode current collector. Thecoating speed was set to 0.75 m/min.

Then, the solvent in the slurry applied to the negative electrodecurrent collector was vaporized in a drying furnace. Vaporizationtreatment was performed at 50° C. in an air atmosphere for 120 secondsand then further performed at 80° C. in the air atmosphere for 120seconds. After that, further vaporization treatment was performed at100° C. under a reduced pressure (−100 kPa) for 10 hours.

Through the above steps, the negative electrode active material layerwas formed over one surface of the negative electrode current collector,so that the negative electrode was fabricated.

A fabrication method of the electrolyte of the power storage battery 1is described.

The electrolyte B2 was formed as follows: vinylene carbonate (VC) wasmixed with a mixed solution in which ethylene carbonate (EC) andpropylene carbonate (PC) were mixed at a volume ratio of 1:1, andlithium bis(pentafluoroethanesulfonyl)amide (LiBETA) and lithiumhexafluorophosphate were dissolved in the solution. The amount ofvinylene carbonate (VC) dissolved in the electrolyte A was 1 wt % in theweight ratio. The amount of lithium bis(pentafluoroethanesulfonyl)amide(LiBETA) dissolved in the electrolyte B2 was 1 mol/L in the molecularconcentration. The amount of lithium hexafluorophosphate dissolved inthe electrolyte B2 was 0.27 wt % in the weight ratio. The electrolyte B2used in this example was formed in the same manner as the electrolyte Bshown in Example 1.

Next, a positive electrode, a negative electrode, and a first separatorwere cut. The size of the positive electrode, negative electrode, andfirst separator were respectively 20.49 cm², 23.84 cm², and 24.75 cm².

Cellulosic fiber was used for a separator. Specifically, 30-μm-thicksolvent-spun regenerated cellulosic fiber (product number TF40) producedby NIPPON KODOSHI CORPORATION was used.

Then, the positive electrode active material and the negative electrodeactive material in tab regions were removed to expose the currentcollectors.

After that, the positive electrode and the negative electrode werestacked with the first separator therebetween. At this time, thepositive electrode and the negative electrode were stacked such that thepositive electrode active material layer and the negative electrodeactive material layer faced each other.

Then, leads were bonded to the positive electrode and the negativeelectrode by ultrasonic welding.

Next, the region where the electrodes are stacked and the lead is bondedwas wrapped with a separator. This can prevent dissolution of the resinlayer of the exterior body by heat treatment performed later and contactof aluminum of the exterior body with the electrodes. The size of thesecond separator was 104 cm².

Then, facing parts of two of four sides of the exterior body were bondedto each other by heating.

As an exterior body, an aluminum film with opposite surfaces coveredwith a resin layer was used.

After that, sealing layers provided for the leads were positioned so asto overlap with a sealing layer of the exterior body, and bonding wasperformed by heating. At this time, facing parts of a side of theexterior body except a side used for introduction of an electrolytesolution were bonded to each other.

Next, heat treatment for drying the exterior body and the positiveelectrode, the separator, and the negative electrode wrapped by theexterior body was performed in a reduced-pressure atmosphere (at a gaugepressure of −100 kPa) at 80° C. for 10 hours.

Next, in an argon gas atmosphere, an approximately 600 μL of electrolyteB₂ was introduced from one side of the exterior body that was notsealed.

After that, the one side of the exterior body was sealed by heating in areduced-pressure atmosphere (at a gauge pressure of −100 kPa). Throughthe above steps, each thin storage battery was fabricated.

Through the above steps, the power storage battery 1 was fabricated.

Then, heat treatment was performed on the power storage battery 1.Assuming that each sample and fluorine rubber are integrally formed asin Embodiment 2, heat treatment was performed in an atmospheric pressureatmosphere at 170° C. for 15 minutes. Specifically, the temperature of athermostatic bath was raised to approximately 170° C., each sample wasput in the thermostatic bath, and after 15 minutes, the power storagebattery 1 was taken out.

Next, the power storage battery 1 was disassembled in a glove box in anitrogen atmosphere, and a positive electrode 1, which was the positiveelectrode of the power storage battery 1, was taken out. The positiveelectrode 1 was washed with dimethyl carbonate (DMC) and dried.

In this manner, the positive electrode 1 was fabricated.

<X-Ray Photoelectron Spectroscopy Measurement>

Next, the comparison positive electrode 1 and the positive electrode 1were subjected to X-ray photoelectron spectroscopy (XPS). The aluminumsurface to which a positive electrode active material was not appliedwas irradiated with X-ray. FIG. 51 shows the position where the XPSmeasurement was performed. The arrow in FIG. 51 indicates the X-rayirradiation direction.

Quantera SXM produced by PHYSICAL ELECTRONICS, INC. was used for the XPSmeasurement. Monochromatic Al Kα ray (1486.6 eV) was used as an X-raysource. A detection area was set to 100 μmϕ. An extraction angle was setto 45°. The detection depth was estimated to be about 4 nm to 5 nm.

FIG. 52A shows spectra of Al2q obtained by XPS measurement, FIG. 52B forC1s, FIG. 52C for O1s, FIG. 53A for S2p, FIG. 53B for Li1s, FIG. 53C forF1s, FIG. 54A for P2s, FIG. 54B for N1s, FIG. 54C for Si2s, FIG. 55A forNa1s, and FIG. 55B for Ca2p. In FIGS. 52A to 52C, FIGS. 53A to 53C,FIGS. 54A to 54C, and FIGS. 55A and 55B, the horizontal axis and thevertical axis represent the binding energy [eV] and photoelectronintensity (arbitrary unit), respectively.

Al, O, C, a trace of F, and a trace of Si were detected from thecomparison positive electrode 1 not subjected to heat treatment. Al, F,O, Li, C, a trace of P, a trace of N, a trace of Na, and a trace of Cawere detected from the positive electrode 1 subjected to heat treatment.

Table 9 shows quantitative values of the elements obtained from XPSspectra. Note that the quantitative accuracy is about ±1 atomic %.Although the lower limit of detection is different from the elements, itis about 1 atomic %.

TABLE 9 Quantitative value [atomic %] Comparison positive electrode 1Positive electrode 1 Al 26.8 17.9 C 25.5 15.7 O 40.4 9.3 S Lower limitof detection or less Lower limit of detection or less Li Lower limit ofdetection or less 5.3 F 3.3 48.3 P Lower limit of detection or less 1.9N Lower limit of detection or less 0.5 Si 4.0 Lower limit of detectionor less Na Lower limit of detection or less 0.6 Ca Lower limit ofdetection or less 0.5

The positive electrode 1 contained less O and more F than the comparisonpositive electrode 1 did. A1 of the comparison positive electrode 1 waspresent in the oxidation state and the metal state, whereas A1 of thepositive electrode 1 was present in the fluoridation state and metalstate. In addition, Li detected from the positive electrode 1 waspresent mainly in the state of LiPFx, LiF, and like. Note that it seemsthat Si detected from the positive electrode 1 is present mainly in theoxidation state and C is present manly in C—C or C—H state.

The above results show that a film containing aluminum fluoride wasformed on the surface of aluminum heated with the electrolyte, which isone embodiment of the present invention. In other words, it wassuggested that a secondary battery including an electrolytic solutioncontaining lithium hexafluorophosphate which was subjected to heattreatment in the fabrication process forms a film on a positiveelectrode current collector, which contributes to prevention ofdegradation of charging and discharging cycle.

Example 4

In this example, examination results of thermal resistance of lithiumbis(pentafluoroethanesulfonyl)amide (LiBETA) will be described.

<Methods of Fabricating Samples>

Three samples of samples 1, 2, and 3 were used in this example.

The sample 1 is described. The sample 1 was lithiumbis(pentafluoroethanesulfonyl)amide (LiBETA). A product by IoLiTec IonicLiquids Technologies Inc. (product number KI-0016-HP) was used forlithium bis(pentafluoroethanesulfonyl)amide (LiBETA). The product in apower state was measured.

The sample 2 is described. As the sample 2, a mixed solution in whichethylene carbonate (EC) and propylene carbonate (PC) were mixed at avolume ratio of 1:1 was used. Lithium battery grade produced by KishidaChemical Co., Ltd. (product code number LBG-00798) was used for themixed solution in which ethylene carbonate (EC) and propylene carbonate(PC) were mixed at a volume ratio of 1:1.

Next, the sample 3 is described. As the sample 3, a mixed solution inwhich ethylene carbonate (EC) and propylene carbonate (PC) were mixed ata volume ratio of 1:1 was formed, and lithiumbis(pentafluoroethanesulfonyl)amide (LiBETA) was dissolved in thesolution. The amount of lithium bis(pentafluoroethanesulfonyl)amide(LiBETA) dissolved in the sample 3 a was 1 mol/L in the molecularconcentration. In this manner, the sample 3 was formed. Lithium batterygrade produced by Kishida Chemical Co., Ltd. (product code numberLBG-00798) was used for the mixed solution in which ethylene carbonate(EC) and propylene carbonate (PC) were mixed at a volume ratio of 1:1. Aproduct by IoLiTec Ionic Liquids Technologies Inc. (product numberKI-0016-HP) was used for lithium bis(pentafluoroethanesulfonyl)amide(LiBETA).

<Thermogravimetry-Differential Thermal Analysis>

Then, thermogravimetry-differential thermal analysis (TG-DTA) wasperformed. TG-DTA was performed with Thermo Mass Photo (manufactured byRigaku Corporation) at a temperature increase rate of 10° C./min underhelium stream (the flow rate: 300 ml/min) from room temperature to 600°C.

TG-DTA results of the samples 1, 2, and 3 are shown in FIG. 56A, FIG.56B, and FIG. 57, respectively. In FIG. 56A, FIG. 56B, and FIG. 57, thehorizontal axis represents temperature [° C.], the left vertical axisrepresents weight change rate ΔW [%], and the right vertical axisrepresents heat flow [μV]. The weight change rate ΔW is weight changerate during heating with respect to the initial weight (before heating).The negative value means the weight decreased by heating.

The weight of the sample 1 started to decrease from around 330° C., andthe weight at around 420° C. was about −95% with respect to the initialweight. In addition, endothermic reaction was observed at around 327.4°C. and around 416.7° C. The endothermic reaction at around 327.4° C. wasprobably caused by lithium bis(pentafluoroethanesulfonyl)amide (LiBETA).Thus, it was found that lithium bis(pentafluoroethanesulfonyl)amide(LiBETA) was stable at room temperature to around 300° C.

The weight of the sample 2 started to decrease from around 150° C., andthe weight at around 235° C. was about −96% with respect to the initialweight. In addition, endothermic reaction was observed at around 232.5°C. The endothermic reaction at around 232.5° C. was estimated to becaused by evaporation of ethylene carbonate (EC) and propylene carbonate(PC).

The weight of the sample 3 started to decrease from around 150° C., andthe weight at around 300° C. was about −78% and at around 450° C. wasabout −100% with respect to the initial weight. The change in weightfrom room temperature to around 300° C. is almost in agreement withbehavior of the sample 2 described above. The change within thistemperature range is thought to be derived from ethylene carbonate (EC)and propylene carbonate (PC).

Thus, it was shown that lithium bis(pentafluoroethanesulfonyl)amide(LiBETA) was thermally stable even in the solvent at 300° C. or lower.

REFERENCE NUMERALS

50: film, 51: film, 52: film, 53: embossing roll, 54: roll, 55:embossing roll, 56: embossing roll, 57: embossing roll, 58: embossingroll, 60: direction, 115: sealing layer, 118: bonding portion, 119:inlet, 200: secondary battery, 203: separator, 203 a: region, 203 b:region, 207: exterior body, 211: positive electrode, 211 a: positiveelectrode, 215: negative electrode, 215 a: negative electrode, 220:sealing layer, 221: positive electrode lead, 225: negative electrodelead, 230: electrode assembly, 231: electrode assembly, 250: secondarybattery, 281: tab region, 282: tab region, 500: power storage device,501: positive electrode current collector, 502: positive electrodeactive material layer, 503: positive electrode, 504: negative electrodecurrent collector, 505: negative electrode active material layer, 506:negative electrode, 507: separator, 508: electrolyte, 509: exteriorbody, 510: positive electrode lead, 511: negative electrode lead, 512:bonding portion, 513: curved portion, 514: bonding portion, 518: bondingportion, 520: separator, 521: bonding portion, 529: exterior body, 700:personal digital assistant, 701: housing, 702: display panel, 703:clasp, 705A: band, 705B: band, 711: operation button, 712: operationbutton, 730: personal digital assistant, 731: housing, 732: leakagedetection circuit, 733: power source, 734: ammeter, 735A: band, 736:electrolyte, 739: functional circuit, 750: power storage device, 751:positive electrode lead, 752: negative electrode lead, 753: exteriorbody, 760: power storage device, 761: terminal, 762: terminal, 771:wiring, 772: wiring, 7100: portable display device, 7101: housing, 7102:display area, 7103: operation button, 7104: power storage device, 7200:personal digital assistant, 7201: housing, 7202: display area, 7203:band, 7204: buckle, 7205: operation button, 7206: input output terminal,7207: icon, 7250: activity meter, 7251: housing, 7300: display device,7304: display area, 7350: display device, 7351: lens, 7351A: image,7351B: image, 7352: frame, 7355: edge portion, 7360: power storagedevice, 7361: positive electrode lead, 7362: negative electrode lead,7400: cellular phone, 7401: housing, 7402: display area, 7403: operationbutton, 7404: external connection port, 7405: speaker, 7406: microphone,7407: power storage device, 8000: display device, 8001: housing, 8002:display area, 8003: speaker portion, 8004: power storage device, 8021:charging apparatus, 8022: cable, 8024: power storage device, 8100:lighting device, 8101: housing, 8102: light source, 8103: power storagedevice, 8104: ceiling, 8105: wall, 8106: floor, 8107: window, 8200:indoor unit, 8201: housing, 8202: air outlet, 8203: power storagedevice, 8204: outdoor unit, 8300: electric refrigerator-freezer, 8301:housing, 8302: refrigerator, 8303: freezer, 8304: power storage device,8400: automobile, 8401: headlight, 8500: automobile, 9600: tabletterminal, 9625: switch, 9626: switch, 9627: power source switch, 9628:operation switch, 9629: fastener, 9630: housing, 9631: display area,9631 a: display area, 9631 b: display area, 9632 a: region, 9632 b:region, 9633: solar cell, 9634: charging and discharging controlcircuit, 9635: power storage unit, 9636: DCDC converter, 9637:converter, 9638: control key, 9639: button, 9640: movable portion.

This application is based on Japanese Patent Application Serial No.2016-234674 filed with Japan Patent Office on Dec. 2, 2016, the entirecontents of which are hereby incorporated by reference.

1. A power storage device comprising: a positive electrode comprising apositive electrode current collector and a film; a negative electrode; afirst separator; and an electrolyte, wherein the positive electrodecurrent collector includes aluminum, wherein the film contains aluminumfluoride, wherein the first separator is positioned between the positiveelectrode and the negative electrode, wherein the electrolyte includespropylene carbonate, ethylene carbonate, and vinylene carbonate, lithiumhexafluorophosphate, and lithium salt expressed by General Formula (G1):

wherein, R¹ and R² independently represent fluorine or a straight-chain,branched-chain, or cyclic fluoroalkyl group having 1 to 10 carbon atoms,and wherein a concentration of lithium hexafluorophosphate with respectto the electrolyte is more than or equal to 0.01 wt % and less than orequal to 1.9 wt % in a weight ratio.
 2. The power storage deviceaccording to claim 1, wherein the first separator includes polyphenylenesulfide or cellulosic fiber.
 3. The power storage device according toclaim 1, further comprising: a second separator, wherein the secondseparator is positioned between the positive electrode and the negativeelectrode, and wherein the second separator includes polyphenylenesulfide or cellulosic fiber.
 4. The power storage device according toclaim 1, wherein the lithium salt represented by General Formula (G1) islithium bis(pentafluoroethanesulfonyl)amide.
 5. An electronic devicecomprising: the power storage device according to claim 1; a band; adisplay panel; and a housing, wherein the power storage device includesa positive electrode lead and a negative electrode lead, wherein thepositive electrode lead is electrically connected to the positiveelectrode, wherein the negative electrode lead is electrically connectedto the negative electrode, wherein the power storage device is embeddedin the band, wherein part of the positive electrode lead and part of thenegative electrode lead protrude from the band, wherein the powerstorage device has flexibility, wherein the power storage device iselectrically connected to the display panel, wherein the display panelis included in the housing, wherein the band is connected to thehousing, and wherein the band includes a rubber material.
 6. Theelectronic device according to claim 5, wherein the rubber material isfluorine rubber or silicone rubber.
 7. A power storage devicecomprising: a positive electrode comprising a positive electrode currentcollector and a film; a negative electrode; a first separator; and anelectrolyte, wherein the positive electrode current collector includesaluminum, wherein the film contains aluminum fluoride, wherein the firstseparator is positioned between the positive electrode and the negativeelectrode, wherein the electrolyte includes propylene carbonate,ethylene carbonate, and vinylene carbonate, lithium hexafluorophosphate,and lithium salt expressed by General Formula (G1):

wherein, R¹ and R² independently represent fluorine or a straight-chain,branched-chain, or cyclic fluoroalkyl group having 1 to 10 carbon atoms,and wherein the first separator includes polyphenylene sulfide orcellulosic fiber.
 8. The power storage device according to claim 7,wherein a concentration of lithium hexafluorophosphate with respect tothe electrolyte is more than or equal to 0.01 wt % and less than orequal to 1.9 wt % in a weight ratio.
 9. The power storage deviceaccording to claim 7, further comprising: a second separator, whereinthe second separator is positioned between the positive electrode andthe negative electrode, and wherein the second separator includespolyphenylene sulfide or cellulosic fiber.
 10. The power storage deviceaccording to claim 7, wherein the lithium salt represented by GeneralFormula (G1) is lithium bis(pentafluoroethanesulfonyl)amide.
 11. Anelectronic device comprising: the power storage device according toclaim 7; a band; a display panel; and a housing, wherein the powerstorage device includes a positive electrode lead and a negativeelectrode lead, wherein the positive electrode lead is electricallyconnected to the positive electrode, wherein the negative electrode leadis electrically connected to the negative electrode, wherein the powerstorage device is embedded in the band, wherein part of the positiveelectrode lead and part of the negative electrode lead protrude from theband, wherein the power storage device has flexibility, wherein thepower storage device is electrically connected to the display panel,wherein the display panel is included in the housing, wherein the bandis connected to the housing, and wherein the band includes a rubbermaterial.
 12. The electronic device according to claim 11, wherein therubber material is fluorine rubber or silicone rubber.